I'll preface this by saying I tried to play Necrons in 5th briefly, and it didn't work out well. To be honest, they need a revision to hang in 5th, as they cannot effectively destroy vehicles.
That being said, the first thing we have to deal with in the Necron Codex is...
'We'll be back...'
This is kind of the necron flavor-rule. Any time a Necron fails a save, it falls over. At the beginning of the next turn, it stands up on a 4+.
Conditions...first, if the unit died to Instant Death or close combat attacks that ignore armor saves, it is ineligble for WBB. There also has to be a similar unit nearby, IE: Warriors need friendly Warriors to stand back up. This means you want two of any kind of Necron nearby, as, say, if you split off some Destroyers to go nail something they won't be able to get back up if they all get splatted.
Lords are immune to that requirement, though.
If you have 25% or fewer of your starting Necrons are alive after WBB rolls, you lose automatically as the Necrons teleport away to repair.
Basically, you need to bring buddies, and bring lots of Necron units to keep your phase-out up, since most enemies WILL have an eye towards phasing you out.
This is the most customizable guy in the codex. He's so-so at melee with an almighty 3 attacks at WS4, I4. He CAN pick up a weapon that ignores armor and invulnerable saves, though. The real notes you need to take on the lord are on the lord's gear. He is T5 with 3 wounds on a 3+ armor save, and he can buy a 4+ invulnerable. He is, though, limited to 100 points of Wargear, and he's also 100 naked.
Veil of Darkness
One per army. The lord and any units attached my deep-strike anywhere on the board in lieu of a move. Popular with shooty units, like Immortals. It gives the army some of its deceptive mobility. It COULD theoreticalyl save you from losing combats, but it won't.
This is pretty much a staple on the Lord. It means you can ignore the 'instant death' and 'ignore armor save attacks in CC' limits on We'll Be Back. It's a short-range AOE, though, with 6". It does apply to the Lord himself, though.
Neat in that it gives the Lord a chance of standing up with multiple wounds back. Doesn't actually INCREASE the odds of standing back up, though.
Gaze of Flame
Everyone loses the bonus charging attack in melee. The LAST place you want your Necron units is in melee, and this won't really do much for you in the long run. There's a faint chance a non-melee unit will fail to beat and sweep you, which might make it useful there.
Glance any AV on a 6. Since most vehicles are AV10 rear, this is usually useless. Since the Lord can take a Warscythe which gives him 2d6+strength armor penetration, well, there's no real point to giving the Lord this.
Lord moves as a jetbike, and goes to T6 where he can't be insta-killed. This would be cooler if the Lord had other units worth keeping pace wit hhim.
Res Orb is pretty much a staple.
Destroyer Body + Scythe = tank hunter; he goes for rear armor and tries to assault tanks to death.
Orb + Veil of Darkness = 200pt lord that drags a shooty unit around. NASTY, but a pricy bit of the army to lose to Mishaps.
Lord + Orb = 140pts. Basic support lord.
the Lord is kind of mandatory. Honestly, I'm inclined either towards the Res Orb (and perhaps Warscythe) lord, or the Destroyer Body model. Most of the rest of the upgrades are pricy and don't really generate a ton of return.
Ignore all saves in melee. T5, 3+ armor save, pack a gun that cranks out S5 shots out to 24". Any enemy within 12" has Ld7. Psykers should never get close to this thing, because they get hosed.
1 attack each. Not Necrons, so no WBB. 36 points a head, and not very durable for that.
They're expensive, and while they LOOK scary in melee, there's no good way to get them there, and they don't get back up. Some folks like to use them as bodyguards for the Lord, but they're just not that effective. You can get the same gun for cheaper on Immortals. Admittedly, there are some tricks you can use with the Soulless special rule involving making people take Ld tests, but it's just not enough to make the unit that cool. Plus, they're stupidly expensive in terms of money.
If you see them, treat them as Immortals save that you'd probably rather shoot them. If they have initiative on you on the charge and you're a small unit, they might actually win; I think a handful of them might actually be able to take Terminators. That doesn't make them good, though.
T5, 3+ armor save. Each carries an S5, AP4 Assault 2 gun that hits out to 24". They are mobile firepower.
Get used to 'dies in assault' on the cons section. WS4, I2, base 1 attack? It's going to lose in melee. T5 is the one saving grace, but it's no defense against a determined assault because Immortals tend to come in smaller units. (5-10, I'd wager).
Immortals will peg infantry and even threaten monstrous creatures with lots of shooting. They can't bag armor. They're not bad at 28 a head; they're probably the top Elites choice in the codex. As such, they're a target worth attending to.
Infiltrate, Deep Strike, Move Through Cover. They have a variety of ways to try to get into melee. Terrifying Visage means that on a failed leadership test, the enemy unit only hits the Flayed Ones on a 6.
...no power weapons. 2 attacks each at I4. When you get down to it, they're essentially assault marines with WBB in lieu of a pistol and jump pack. They can't take out anyone with armor, and they can't take out anyone with T5 or better, and they can't reliably get the charge. Also, S4 on rear armor 10 means they aren't killing vehicles.
...A bit useless, really. They need some actual killing power in melee, and the only way you could get a power weapon in there is with a Lord, who's good points after bad and removes the unit's special deployment options. Beyond that, they're a footslogging unit that wants to get into hand-to-hand, and that's a big problem.
10-20 in a squad.
To be honest, they're basically I2, Ld10 bolter marines that cost a minimum of 180 points for the minimal 10 guys. You NEED them to get numbers up, but the guys really just aren't that good.
They CAN glance any armor on a six, but glances won't kill anything outside of open-topped vehicles. They are also footslogging, meaning that they're either moving 6" and shooting 12", or standing still and shooting 24" once. They HAVE to sit on your objective, but they also have to get to other objectives.
Better yet? They die in assault. I2, Ld 10, one attack each at WS4, S4. If they get assaulted, they're going to lose, and then they're going to get swept, and that's going to make WBB a bit more difficult.
And better than that? You're taking 20 of them in your army, minimum. To be honest, the troops flavor the army (since you're taking 2-3 of them), and Necrons at this point do not have a lot of options for troops. Warriors just have problems doing much because they are slow, shorter-ranged, and have guns that are basically special glancing bolters.
S6, I6, 3 attacks each, 3+/3+ saves. Move like jetbikes, have frag grenades, and move through terrain.
41 a head, max three in a unit. Plus, each Wraith is $20.
Wraiths are pretty nasty, BUT they are expensive and come in small units. There's a decent chance that they'll get killed off before they get where they're going. Furthermore, a good set of saving throws totally neglect the few wounds they crank out with only WS4 to go with that. Wraiths also compete with Scarabs and Destroyers for Fast Attack slots.
Without a doubt, they're your best Fast Attack choice.
BS4, each carries a 3-shot, S6, AP4 gun with a 36" range. Moves like a jetbike; may move and shoot.
Model is $20 a head, you want 6-10. T5, 3+ save, a single wound. MUST be taken in numbers to survive. Like most Necron units, you want two units of Destroyers near each other so the enemy must level all of them in a turn in order to deprive you of WBB rolls.
These guys are adept at wounding infantry and lightweight (AV10-11) vehicles. They're mobile guns, and good at it. Using them is a significant investment in points, as the two minimal 3-Destroyer units cost 150 each. I would suggest units of 4-5 just for sake of durability.
These are your distraction units.
LOTS of wounds. 12pts for a 3-wound base. Fearless. Moves like a jetbike. Gets a 3+ cover save when you turbo-boost them at something. You can buy Disruption Fields so they can try to swarm vehilcles.
Not really going to kill anything other than Grots, but it can slow down anything that can't insta-gib them. WS2 and S3 means that even with 3 attacks, they're not going to kill much, especially when the unit gets worn down. They are also vulnerable to Instant Death; S6+ hits drop a scarab base outright.
They're a tarpit; no more and no less. No Retreat! is fairly harsh on them, so don't expect them to actually kill much. Theoretically they could get that 'glance vehicles on 6+' deal in Disruption Fields, but you're better off with more scarab bases, since usually they're best going after units.
The biggest thing it does is remove the distance requirement for WBB. They are also S6, T6, Fearless, and can get a gun. Additionally, they can crank out a Scarab Base, and thanks to the rules so long as there's only one scarab base, it's ablative wounds. However, it's also only going to soak a few wounds due to wound allocation.
WS2 with three S6 attacks in melee. Can't really shoot, either, since it can get a couple anti-infantry shots at BS2. Creating scarabs can sometimes be fatal.
The utility comes in the whole WBB alteration. One to two is enough for your force, if any.
Pros: T5, Jetbike, BS4 Lascannon equivalent
Cons: 1-3 per unit.
Overall: It's the ONLY thing in the codex that can effectively deal with AV greater than 11. You can't get a lot, and you might want to consider a Tomb Spyder to keep them coming back. They're almost essential for delaing with the heavy armor, BUT, they have a 36" range and STILL have to pen and roll the damage results you need. At BEST, your army is cranking out 2 hits on 3 targets, which won't kill them all. Still, it's the only real way to get penetrating hits on tanks in your army, and in larger games I'd say you should consider them, or prioritize killing them.
AV14 all around. Nasty shooting; can deep-strike. Ignores melta and extra dice against armor penetration. Ordnance shot. Can teleport troops AND give them a re-roll on WBB. May Deep Strike, and reserve troops through that. Fires d6 shots at EVERYONE within 12".
235 points of 'I'm not a Necron and I don't count towards phaseout.' While they HAVE an ordnance shot, you can't use it if you're using the 'lith to make your necrons more durable. Deep Striking is risky and denies you use of them for some time. People are used to gunning down AV14, to an extent. Taking multiples eats into your points. Only gets to move 6" a turn.
Additionally, the model is HUGE. You aren't going to hide it.
The 'lith is hellishly expensive. it CAN make your force more durable and give you options, and it is durable, but in practice it's going to crank out close-range shots IF it gets there, and generally make your guys a little more durable by washing them through the portal each time.
These are 300 and 360pt characters you can take as HQ units. They are hellishly powerful, BUT have some serious drawbacks.
C'Tan in General
Ignore terrain. Ignore armor saves. Ignores invulnerable saves. Explode when killed. Fearless. 4+ invulnerable save. T8. 5 wounds. Monstrous Creature. In other words, it loves special rules and has a lot of them. I mean, it's a god walking on the battlefield. Oh, and if for some reason a Callidus Assassin tries to shank a C'tan with a C'tan Phase Blade, the C'tan absorbs it (...not that it matters, because the C'tan would promptly snap the Callidus in half).
The Biggest Con
C'tan are TALL. I'm talking like a 4" tall model. It CANNOT really hide. It doesn't really need to worry about getting cover saves with a 4+ invulnerable, but the only thing that can realistically shield it is the Monolith. Cover saves don't matter because they have a 4+ invulnerable anyway.
360 points for this guy. S10, 5 melee attacks, WS6, I4, a 24" lascannon shot. His special abilities deal with hordes. 'Gaze of Death' slaps a large blast template over him and everyone under it eats an S4 hit with no armor saves. Etheric Tempest blows back folks with unmodified strength below 4.
The Nightbringer is a wrecking ball, pure and simple.
300 points. Only S9, WS5, but I5 as well. His abilities are more tactical in nature, as opposed to the Nightbringer's "I AM THE GRIM REAPER OF DEATH". Decieve says 'Hey, you, take a pinning or morale check even if you normally auto-pass that.'
Grand Illusion means the Necron Player can roll to re-deploy units at the beginning; on a 4+ the unit they designate gets to redeploy and they get to roll again. The whole ARMY can get redeployed like this.
'Dread' means that a unit takes a leadership test and if it fails they only hit on a '6' in assault. The Deciever cannot actually be in assault when he does this, though.
'Misdirect' means that the Deciever can fall back from an assault at will; the enemy cannot consolidate back into it.
Can you do the C'tan C'tan?
If the enemy can bring a lot of S7+ fire to bear, the C'tan is going to die. You don't want to know what Lootas do to your 300+ point model. A 4+ save is NOT going to keep it from eating wounds, and T8 only protects you against so much. The enemy KNOWS you want to get that thing in melee because they KNOW the C'tan is absolute death in melee against just about anything. The enemy also knows that the C'tan moves 6" a turn, so the C'tan is a bit ponderous.
The Deciever has a lot of neat options, but still, it's slow and not going to get into the assaults it wants to. In general, it's the only real CC threat in the army, and it's expensive. I'm not sold on its effectiveness, because I've taken them a few times and they draw fire and explode violently. There just aren't enough targets that REALLY want your high-strength fire.
Deficiences in the Necron Codex In General
There are simply no good ways to KO vehicles for the Necrons. Destroyers can do well enough vs. AV10-11, but something like a Dreadnought is a death sentence for anything in the army. The only things that might take out vehicles in melee are Wraiths, Destroyer Lords with Warscythes (for the 2d6+5 AP), and C'tan (S9-10+2d6 = Ow).
Triple Land Raiders is going to make Necrons Cry. Even light mech (lots of rhinos/immolators) is going to give the Necrons trouble.
Necrons have no good assault units outside of C'tan. Scarabs can sometimes hold things up, but that's so you step back and shoot the unit. Flayed Ones are a joke, and Wraiths could be lethal if they were cheaper and more numerous. Pariahs WANT to be assaulty, but just can't do enough damage even if they are S5 and ignore all your saves.
Plus, the bulk of the units are I2. They are going to lose, and they MIGHT get away from Orks.
The Necrons have some good anti-infantry units, but as I've said, they just can't take vehicles out with massed glances. They die in assault, and We'll Be Back is a bit complicated. I like the fluff and all, but the army just can't reliably hang in 5th.
If you're playing Necrons I salute you, as you've an uphill challenge against anyone who brings vehicles.
If you're playing against Necrons, I recommend you blow away anything that's got a Destroyer body, then prioritize anything with the Necron special rule. Do be worried about C'tan and Monoliths if they get close to you, though.