If Elites are the least useful slot in the Necron armory, Fast Attack is perhaps the most misunderstood. For many players, FA just means spamming Wraiths or Scarabs- and to be sure, these are both good units- but the truth is almost every choice in the FA slot is not simply viable, but actually very useful. However, which ones you want and why will be greatly influenced by the other choices in the army- a common theme for Necrons, as they lack the sheer, derp-obvious power of some other codices.
Canoptek Wraiths
Wraiths made a HUGE leap in effectiveness from the old book- like almost everything, though for some reason a lot of people were convinced that Wraiths were good before. (Protip: they weren't. S6 Marines on Bikes? For a ridiculous number of points? Sorry, no.) However, Wraiths now bring a good combination of melee threat and defense to the army and can have excellent synergy with many other choices in a list, and the only thing they really lost was WBB (and some thirty points off their cost.)
With S6, W2, A3, and a 3++, Wraiths come out of the gates looking pretty scary to most things; sure, they only have I2, but jump infantry movement, ignoring terrain completely, and Rending all combine to make that almost a non-issue. Further pushing the envelope are their options- you can wound-differentiate the squad by adding Wraithcoils, which oh yeah incidentally make sure that you're attacking first as well, something of a miracle for Necron units. They can also add a shoddy little pistol to their equipment, but frankly it doesn't impress, so maybe take one to break up the wounds a bit more and that's it. The Beamer is total garbage and should always be ignored. Oh and on top of everything else they're incidentally Fearless, preventing your low Init from becoming a major liability.
Wraiths can serve a couple different purposes in an army, one of which being magnifying other assault units' potential- when Lychguard or Spyders are swinging before the enemy, they are worlds more dangerous. They can also serve as a tarpit unit to hold up enemy melee threats- with their 3++ they can hold off almost anything except Terminators for a good long time and will be able to inflict non-ignorable casualties in return. They can partially fill a counter-assault role, but for all their advantages they simply aren't a devastating enough threat to really do that job all by themselves, so you are almost certainly going to need something else to help them, pushing them back into job #1, above.
Many players will try to push a "Wraithwing" army that fields large numbers of them and uses them in an offensive manner, aggressively assaulting the enemy's position; don't do this. As already stated, Wraiths are NOT a top-class assault unit- they will be mediocre stuff like Tactical Marines and can even do some damage to tougher units, depending on their setup, but they aren't really able to handle a lot of the potential threats out there- and moreover they're expensive, coming in at near-Terminator prices. If you're fielding 15+ of them, you're dropping a LOT of points, which will leave you very vulnerable elsewhere- and once those Missiles, Lascannons, and Meltaguns start raining down on them, they will collapse fairly quickly. Necrons are NOT an assault army and trying to make them into one will, more often than not, result in failure. It's fine to cover your weaknesses, but it's just as important to play to your strengths.
Wraiths are more durable than Scarabs in melee, but less so against shooting; they're more effective against infantry/etc, but less so on vehicles than their counterpart. As a rule they'll require a larger investment than Scarabs, but will be more able to handle different targets and get to where you need them (as Jump Infantry and ignoring terrain are better than Beasts for getting about.)
Canoptek Scarabs
Guess what vocabulary word Mr. Ward had to use in the codex this time around? Scarabs are one of the hot new celebrities of the new book, and with good reason- their Entropic Strike is absolutely devastating to slow-moving vehicles and they are quite resilient as well as relatively cheap. However, they have also been the victims of a lot of hype, touting them as a must-have for armies or the end of mechanized lists. Neither of these things are true.
Scarabs bring three Fearless wounds and four Entropic attacks to the party, along with WS2, S/T3, and the rest of the standard Necron statline. Like the other Canoptek units, they do not have Resurrection Protocols. This means a couple important things: being Swarms, they will usually have a 3+ cover save, making them extremely difficult to remove with many kinds of firepower. However, they do NOT have Eternal Warrior, so S6+ shots that go through will take out significant swathes of the unit; blasts and templates can do the same, with the latter being particularly deadly because they ignore cover. (Scarabs fear nothing more than an Incinerator or Inferno Cannon, which kill two bases for every successful wound.) Their generally weak combat stats also mean they are somewhat middling at killing standard Marine targets- they can drown them in attacks/bodies, but will likely take a fair number of casualties in return, since the enemy is striking first and they must contend with No Retreat. However, this doesn't stop them from being an excellent tarpit when one is needed- indeed, with Beast status they have the potential to assault a lot of backfield targets rather quickly.
Scarabs will most commonly be brought to a list to deal with vehicles, though, since that is where they excel. Any list that expects to be Immobilizing or Stunning a lot of vehicles- including anything with extensive Tesla, Tremorstaves, Voltaic Staves, etc, has good reason to want some Scarabs, as they can quickly pounce on such units (or anything that chooses not to move) and wreak terrible havoc on them. Expect to be largely wiped out in return following such an assault, however- at short range, often with no cover and in the teeth of the enemy's guns, your Scarabs will die in droves. For this reason, Scarabs are often a suicide unit, but this isn't all bad- even just having a small contingent of Scarabs (4-6) in an army can present a very real threat to the enemy that will keep their parking lot moving and reduce their firepower, all for a pretty reasonable price.
It is also worth mentioning the "Scarab Farm" here- that is, multiple Spyders (or units of Spyders) pouring reinforcements into squads with the intent to use them aggressively to tie up the enemy. In the general sense this is fine; they're both good units and the synergy between them is excellent. However, as a grand strategy- that is 3x3 Spyders and a large number of Scarabs the plan is very vulnerable, denying you as it does access to many other units and committing you to an all-or-nothing assault.
Tomb Blades
Of all the new units these have gotten perhaps the least attention, rules-wise, and I suppose that's not surprising; I admit I was skeptical of them myself when the book first came out, but they've since grown on me a lot. To start off, one must remember to look at them the right way- they're cheaper than a Space Marine bike and come with a MUCH better gun and a little worse save (but RP can make up for that.) They also can come in units of one, which is important, as we'll see in a moment. Swapping for Gauss is probably not a good plan- S5 will still glance most transports and the free hits from Tesla outweigh the possibility of getting into Rapid Fire range, which you will almost never want to do. (Remember, though, jetbikes are Relentless.) The Particle Beamer, on the other hand, can be a pretty good choice, and we'll look at it some more in a moment here. Nebuloscopes are silly; Shield Vanes are inferior; Shadowlooms can be useful, but do keep in mind that they are a major price bump.
So what's a Blade good for? Well, one option is if you have a free FA slot and some points to spare, just suit one up with a Particle Beamer (and Shadowloom, if you're feeling fancy) and have it run around your side of the field annoying things. S6 isn't very good against transports, but the moment you start disembarking units, that small blast that wounds on 2s is gonna be annoying, and it can pepper fire support units (Long Fangs, etc) in the interim as well. In KP missions you can simply jet them around whenever there's any danger and in any other mission they're enough of a threat to be useful but too small to really direct firepower at, which is a nice place to be sitting. Squads of Tomb Blades carrying simple Tesla can also make a good harassment unit- they get you a surprising number of S5 hits on the move and can contest and such late game.
Tomb Blades are a little more narrow than Wraiths or Scarabs, but they also strengthen your primary strategy (shooting) rather than investing into defense against a weaker one (melee.) For those looking to diversify their threats, they can be an excellent, mobile choice.
Destroyers
Destroyers, not particularly good before (and again, people will try and convince you otherwise, but it's best not to listen to them), unfortunately got hit pretty hard with the nerf bat. They lost the ability to turbo-boost, which is largely inconsequential, but their gun also lost 12" of range, a strength, and a shot, all of which are very painful for them. As it stands, regular Destroyers just don't have much of any use at all in the codex.
Heavy Destroyers, their cousins, on the other hand, are not nearly as bad off as many think. You are limited to three per squad, but really that is not going to be much of an issue- like Tomb Blades, they will often be a "gap-filling" choice anyways. They remained essentially the same, gaining Preferred Enemy (but should never be in melee) and otherwise being pretty identical. This means they are reasonably accurate with a long-range gun with high strength- something unusual in the Necron army. T5 and 3+ also make them no pushovers to bring down, either, especially at the ranges they operate at; combined, you have a somewhat tough and mobile anti-tank shooting platform, an interesting counterpart to the infantry-killing of Tomb Blades.
Heavy Destroyers, as I said, will usually be taken to fill the last points in a list, although their cost means "last points" may be a bit of a misnomer. At 60pts per body they certainly aren't cheap, but by keeping them solo you can also make them a very poor target for the enemy's firepower- only a few guns will have the range to reach them, and those that do will often struggle to get past T5, 3+ armor, and 4+ cover combined. And if they do? Well, better aimed at your Heavy Destroyers than your other infantry squads, transports, etc- every Lascannon shot that goes to them isn't going elsewhere. This becomes especially valuable in Necron MSU lists that can field in excess of fifteen "major" targets with relative ease, all of them capable of projecting significant firepower. Seen in this light Heavy Destroyers may not be an auto-pick, but they serve their purpose well enough and shouldn't be dismissed as merely worthless.
