Now, at the core you're trying to win your games of 40k. There are three basic mission types; two involve claiming objectives and the last involves simply killing the other guy.
1) Capture and Control
Two objectives. This is essentially 'capture the flag,' as objectives are placed after deployment is determined.
2) Seize Ground
2 + d3 objectives, or 3-5 objectives. We call this the 'objective mission', and it's not guaranteed that there will be objectives in your deployment zone.
Kill the other guy. This is the 'kill points' scenario, or if you're willing to look into a more balanced system, Victory Points.
Addendum: in case of a tie on objectives, annihilation provides the 'moral' victor.
So, what does this mean to you? Your army must be able to controlling objectives and blow things up, and do both of those reliably.
As objective missions work, you need only control more objectives than your opponent at the end of the game to win. Let's take a look and see what this means, because there's more than one way to pull this off.
If you want an objective to be 'yours', you must have a Troop unit within 3" of the objective marker, and no enemy units in that area. Good news: troops are compulsory. Better news: troops can often take transports, and if the transport is within 3" of the objective, you're claiming the objective.
When claiming objectives with troops and you do not have the bottom of the turn, it's advisable to put the exit near the objective, so you can at least disembark and still be within that crucial 3" zone.
Troops and Controlling Objectives
You are required to pack two troops units into your army. What is the 'right' number of troops? To be honest, this question depends from codex to codex. Some codices have some very nasty troops (IE: Chaos' cult troops, Imperial Guard vets w/ meltas in Chimeras...) and some do not. At a minimum, though, once you hit 1500 take at least 3 scoring choices. I say 'scoring' because some troops choices are swarms, and as such cannot score. (IE: Nurglings, Ripper Swarms).
In truth, the '3' is a suggestion, and depends on the durability and mobility of your troops. Durability comes in the form of numbers (...a 30-40 man Combined Infantry Squad of guard with a Commissar can survive a lot solely by having a crap-ton of wounds and Stubborn Ld 9), special abilities and/or above average stat-lines (Plague Marines are T5 with a 3+ armor save and Feel No Pain; Wraithguard as troops are 10 guys at T6 with 3+ saves...). Mobility for troops usually comes from being mounted in a transport, but may also be simulated via Deep Striking (for daemons, or with Drop Pods). Note that there's a difference between being in a transport, and being mounted in single-shot deep-striking deployment; the latter is only 'mobile' until it's used.
Should someone skimp on troops, then killing all their troops is a legitimate strategy. You cannot control objectives without troops. Losing all your troops means you're playing for a draw, 2/3 of the time.
The other side of the 'objective' coin is to deny the enemy the ability to put troops on the objective. Obviously, dead troops don't hold an objective (well, if it's a small objective, you might have to pry it from their rapidly-cooling hands...). You deny objectives by putting a 'contesting' unit on it. This unit can be anything from another troop unit to something as annoying as an empty Rhino transport.
What does this mean to you? Nothing in your army that can move is useless. So, maybe they nuked the tactical squad that came out of the Rhino. But, you can still move that Rhino near the objective. Something like Terminators sitting on an objective may not claim it, but they will be difficult to shift off it.
Additionally, there's a time sense to claiming objectives. Until turn 5-7, it does not matter who is on what objective. If you can put something on objectives the enemy does or might control by this time, you can force them to change plans accordingly. Fast contesting units are not vital (and indeed, not everyone has access to them) but you can still do this. I keep using Rhinos as examples because once a Rhino is empty, it's usually less a priority.
Mobility and Objectives
Ok. First off, get mobility in your army. Not all armies are equally mobile (IE: Eldar have Fast Skimmer Tanks. Marines...have regular transports. Guard? Have transports that move even less, and then shoot). Doesn't matter. You need a way to get your troops to any objectives on the field, and you cannot rely on always having objectives in your end.
In other words, mount up! Even if you get one turn of movement out of a regular transport, your troops still got 12" of movement, and had to be shot out of their transport. Every turn troops are in transports is a turn small arms and other troop-killing weapons aren't killing them.
Mobility also plays into objective denial. If it's bottom of the 5th turn or later (or, if you know the turn is the last, like turn 7, or there's a time limit in the tournament) then plan accordingly. Make the enemy shoot something off his objective. For example, in the 5th (and lucikly, last) turn of one of the games I played, I threw Fire Dragons at the Guard player. Their Wave Serpent ended up within 3" of his objective, and the Dragons nuked a Leman Russ. My opponent had no choice but to try to kill the Wave Serpent, as it meant he was no longer in control of his 'home' objective. If you want someone to talk more at length about this, go check out Fritz if you haven't already.
Objectives Basics Summaries
-Keep your troops alive; 3+ choices at 1,500pts or above
-Troops claim objectives, but anyone can deny them
-Have the means to move troops to objectives (BRING TRANSPORTS)
The Business of Destroying the Other Guy
Blowing up the other guy is the primary objective only 1/3 of the time, but it's always part of the game. After all, he needs troops alive to hold objectives, and the opponent generally wants to keep you from doing the same.
There are three basic types of units out there: Infantry, Vehicles, and Monstrous Creatures. Infantry claim objectives, vehicles kill things and move objectives, and monstrous creatures tend to stomp around and wreck things. MCs are basically wound-based walkers, when you get down to it.
Your list just needs to be able to kill infantry. You will always face enemy infantry. So, what kills infantry?
Just about every last basic trooper has a shooting attack. Space marines, for example, carry bolters. A full tactical squad can crank out a decent number of shots. Eldar Dire Avengers carry Shuriken Catapults, and fire at enemy troops. Pretty much every troops selection in the game has some kind of firearm (outside of most Daemons and Tyranids). With small arms, though, you need quantity to really see results, as people tend to be in cover.
Cover means everyone gets at least a 4+ save, you're checking to hit, you're checking to wound, and then they're taking saves. If you can deny the other guy cover and beat his armor save, you're in good shape. If it's something like marines or someone else with a 3+ save, then at least using something like plasma downgrades their save to a 4+. It helps, but you should ask yourself if it's worth the cost or if you have other options available.
Bottom of the line? Small arms tend not to have great range. You will need to get close to use them, or the enemy comes to you.
NOW we're cooking with napalm (or alien equivalents). Cover keeps troops alive. Flamers negate cover. Flamers ignore your ballistic skill, and just need to get close. It's pretty much essential to bring them on mobile platforms, like speeders or out the back of transports.
Why template weapons, though? The basic Flamer is S4, AP5. Lightly armored troops like Guardsmen and Guardians and Orks die when wounded by this, because they get no save. Pretty sweet, isn't it?
Heavy flamers make it better. On a 2+, 99.9% of non-Marines simply DIE when under a heavy flamer template. S5 AP4 does that. There are some non-marines out there with 3+ saves (Eldar Scorpions and Spiders, for example) but not many.
-Heavy Weapons, Blast Weapons
Some heavy weapons can pull this trick off. High-rate-of-fire weapons like Heavy Bolters and Autocannons can heap wounds onto infantry more reliably than small arms, by virtue of higher strength. Missile launchers and other blast weapons can do this in a pinch, but cover and a good spread on infantry units mitigate the damage. Tools like GF9's TAC Template are amazing aids for this. What's that mean to you? Blast weapons CAN work and don't rely as much on ballistic skill, but their damage can be minimized. (Considering that the TAC template costs like $6, you should seriously go out and get one. Just do it, it's amazing what help it is vs. blast weapons and the like)
Troops also tend to die when hit with melee weapons. Note that assault is heavily dependent on the unit. Not all codices have access to decent assault units (IE: Tau, Guard...). In this case...you don't assault. But, some assault units (IE: Thunder Hammer/Storm Shield termies) are worth it for their sheer lethality. Assault is also a mop-up option; if neither of you are assaulty choices and you can hit a weakened unit with a charge, it might be worth it.
The problem with assault? If you do it too well, you end up out in the open on the next turn, and the enemy can simply shoot you to death. It can and will happen if you are not careful. Some units are more resistant to this subsequent shoot-to-death 'fest, like Hammer/shield termies with a 2+ armor save and 3+ invulnerable, or 3+/4+ with re-rolls on a Seer Council (who will first crank out lots of heavy flamer attacks, THEN assault you to clean you up.)
Killing Vehicles and Monstrous Creatures
I'm lumping these two together becuase for the most part, the same thing kills them: high-strength shots (with good AP against Monstrous Creatures).
AP1 gives you a crucial +1 bonus when rolling damage results. The average penetrating hit will annoy (shake or stun) a vehicle 33% of the time, knock off a weapon/immobilize it 33% of the time, and kill it 33% of the time. Melta weapons shift this to 'annoy 16% of the time, damage 33% of the time, kill 50% of the time.' Bottom of the line? AP1 is highly useful. However, it tends to come with the 'melta' special rule...
Melta is almost always combined with AP1, though some AP1 weapons are not melta (IE: Tau Rail Guns). Melta means that within 1/2 range (6" for meltaguns, 12" for multi-meltas) each hit gets +2d6 penetration instead of +1d6. Most melta weapons are strength 8, and 2d6 averages 7, so on average a melta hit will penetrate a vehicle. On average.
Melta weaponry MUST be combined with mobility to be of any use. It's not very hard to dance out of 6" range when the other guy is walking 6" a turn.
Ordnance rolls 2d6 and takes the higher roll when penetrating vehicles. As such, ordnance is a legitimate threat to armor. Higher strength is always better, as a Demolisher Cannon (S10) will crack armor more reliably than a Battle Cannon (S8). If you want a rubric of the odds, check out Jabberjabber's table. Sadly, ordnance weapons are seldom combined with AP1. They are also Blast weapons, and as such may scatter off the target. If the center dot isn't on the target, then it's no good: you only get half-strength. Additionally, some models like Dreadnoughts and Killa Kanz have a small area to land the center-dot on.
These usually include missile launchers and lascannons. For the most part, these tend to not come in sufficient volume to do damage to armor when needed. They penetrate light and medium-weight armor (AV10-12) well enough, but cannot usually bring enough shots to reliably kill AV14, and sometimes 13. The Imperial Guard Vendetta, with its 3 twin-linked BS3 Lascannons, is a notable exception. Ultimately, though, killing AV14 is pretty much the province of the melta weapon, as it penetrates more reliably.
These guys occupy a special niche: they often come with a decent rate of fire and long range. The typical example is the autocannon (S7, AP4, Heavy 2, 48" range), and the Scatter Laser (4 shots, S6, AP6, 36" range). They are not superb at killing armor (that's the melta weapon for you), but generally have the right combination of range, strength, and rate of fire to kill transports. They are by no means a substitute for melta weaponry, but rather a compliment for knocking transports and light tanks down. S6 often comes in greater rates of fire than 7, but it's a matter of what your army can get.
-S5 or lower weapons
These are still strong enough to wound monstrous creatures reliably, but are not exactly top-tier anti-tank weapons. S5 is still sufficient to take out AV10, but requires luck. S4 troop guns can theoretically glance AV10, but this is not exactly the most efficient use of them.
Assault is an option for killing vehicles. Most vehicles have AV10 on the rear armor, which means offensive grenades (IE: Frag Grenades) can damage them, as can some infantry. Any infantry unit with Krak Grenades or a powerfist can likewise harm them, and Dreadnoughts (and other walkers, and monstrous creatures) more or less automatically penetrate them. Vehicles defend against this by postioning themsleves so as not to get assaulted readily, or by moving and thus reducing the chance to hit. You hit automatically if they are stationery or immobilized, on a 4+ if it's a 6" move (or Combat Speed), and on a 6+ if they moved at a higher speed.
Assaulting Dreadnoughts and other walkers is generally inadvisable, as they are difficult to damage with just one powerfist. Grenades need 6's to hit unless the walker is immobilized. Killing Dreadnoughts in melee requires specialist kit; you need to hit them multiple times at high strength. Something like Thunder Hammer/Storm Shield terminators (with massed S8 thunder hammers) can do it, as can Eldar Seer Councils (everyone there has a Witchblade, which hits vehicles at S9). Large squads of marines with powerfists might be able to do the trick, but at a cost to themselves, since they're looking at usually 2 powerfists attacks, one hit, and then a 33% chance of penetrating the AV12 walker.
This is different for something like a Sentinel or War Walker or other AV10 non-melee walker. They are not nearly the same threat, with usually a single attack that does not ignore armor saves. These units can, in a pinch, bog you down, but not necessarily kill you.
-Addendum for Monstrous Creatures
Most monstrous creatures have a Toughness of 6, four wounds and a 3+ armor save. Accordingly, you care about AP enough to bring something AP3 or better. Monstrous creatures are actually more durable against melta weapons than vehicles, since a hit from a meltagun can pop a vehicle, but might only wound the monster. Generally, though, the same things that can kill vehicles can reliably wound monstrous creatures. Powerfists and other high-strength and/or armor-ignoring melee attacks can work on them, though bear in mind the close combat attacks of monstrous creatures ignore armor saves.
This gets into list-building more directly. So, we know we need to be able to move towards objectives, and we need to bring weapons to harm any potential threat on the board. What does this mean to you?
What's redundancy? Bring multiples. If you have one troop unit mounted in a rhino, and that's your attempt to claim any objective you can't deploy on or walk to quickly? I blow it up, and you're in trouble. If you brought one meltagun and I have three Land Raiders? You're screwed. Some people consider redundancy 'boring', but to be honest, effectiveness needs redundancy. The enemy should not be able to kill one unit and necessarily eliminate your entire capability for doing something. Bring multiple flamers, bring multiple meltaguns, bring multiple transports!
It's a little secret to having an effective army list. Bring several selections capable of laying down long-range fire. Bring a couple ways of killing tanks, bring a couple units that can kill infantry, bring some units that can contest late-game.
Sounds like a lot of units, doesn't it? It doesn't have to be. There's another important concept:
Not every army does this; case in point my beloved Eldar. Dire avengers will only ever effectively shoot enemy infantry. However, for the Eldar? Dire Avengers can take a Wave Serpent, and suddenly this troop configuration can: move 12" a turn and fire weapons. My Wave Serpent packs a Bright Lance, giving it a decent heavy anti-tank punch at range. It packs a shuriken cannon, and being a Fast vehicle it can move 6" and fire both guns to be able to harass lighter armor, monstrous creatures, and even infantry. It's just a versatile unit.
Other examples of duality include...
-10-man Tactical Marine squad w/ Flamer, multi-melta, Rhino (205 points)
It can shoot up infantry with bolters and the flamer. It can threaten heavier armor with the multi-melta. With the rhino, it can fire the multi-melta out of the fire point. It can move, it can block LOS with the rhino chassis, it can score with the marines and the empty rhino can contest.
-10-man Imperial Guard vet squad w/ 3 meltaguns; Chimera w/ Multi-Laser/Heavy Flamer
Net cost? 155 points. The vets within are consummate armor-killers. The Chimera protects them, grants them mobility, and provides long-range harassing fire and short-range infantry-killing power.
-Multi-melta/Heavy Flamer Space Marine Land Speeder
Need to cook troops? Heavy flamer. Need to kill armor? Multi-melta. It has the speed to put its short-range weapons to work.
-Marine Predator w/ Heavy Bolters/Autocannon, or Eldar Falcon w/ Pulse Laser, 2 Shuriken Cannons
Both of these are similar; they crank out a heap of shots to harm infantry at range, or fire the 'big gun' at armor. The Falcon has the added benefit of being able to shelter a small troops unit and sit on an objective.
The previous are just SOME examples. They're what I'm familiar with, but my point is that you can and ought to configure your units to have multiple capabilities.
Whew. That felt hefty, but let's condense it. When building a list, bases you should cover:
Get troops and weapons where you need them
Kill the other guy's troops, AND vehicles, and do it well
Can you secure your own and deny the other guy theirs?
Do you have multiples for your stuff? More than one tank-killer, infantry killer, etc.
If your codex allows it, have you made your units as versatile as possible?