So, this is an unusual combo, but according to the 7th edition Ally Chart, Necrons and Tau are Allies of Convenience, meaning they are for all intents and purposes “neutral” when taken in the same army list (i.e. no real mutual benefits, but no major drawbacks).
-Two squads of 6-8 Necron Triarch Praetorians
-One Tau Y’vahra Battlesuit (with 2 x Missile Drones)
So, starting with the Praetorians, they are tough (T5, 3+ save, Reanimation Protocals), have good mobility (Jump Troops, so 12″ move and can Deep Strike), have respectable shooting (One S5 AP2 shot at 12″ pe rmodel) and quite good CC ability (3 x S5 AP2 attacks at I2 on the charge). Overall, they are quite well-rounded and can fulfill many different roles well, which is part of the reason I think they partner with the Y’vahra so well.
Now, looking at the Y’vahra’s rules, you can see that it is solid in CC (2+/4++, potentially gong up to 3++ with Nova Reactor, with 4 x S6 AP2 attacks on the charge due to its status as a MC), but where it really excells is close range shooting. With the ability to potentially drop 2 x S6 AP2 Torrent templates (at 6″, vice normal 12″ Torrent range) and up to an additional 6 x S8 AP3 shots (with Haywire rule) at 12″, this Battlesuit can be devastating after it Deep Strikes or after it uses its special “quasi-Monstrous Creature Swooping” rule conveyed to it by its Vectored Thrust Array wargear.
So, how do the Praetorians and the Y’vahra work together? Well, you can start them all on the board and then use their Jump Pack/Jet Pack status to move them rapidly up the board turn 1 to set up for turn 2+ shooting and assault. Alternately, you can start them all in reserve and then DS them in turn 2+ together to overwhelm a chosen target. As I look at it, use the Y’vahra shooting first (especially the Templates), then clean up whatever is left with the Praetorian shooting. After that, the following turn you can shoot some more and then charge or, if your opponent is foolish enough to charge himself, this batlegroup should be able to handle most units pretty handily as long as they “gang-tackle” them (also, the Y’vahra has Hit and Run if it does need to escape combat for some reason).
Statistically, just to illustrate the potential damage output of this proposed battle-group, let’s do some Mathhammer to see how their shooting would fare against one of the more common “deathstar” units in the game right now, a squad of 6 Centurions with a 5++ cover save and Endurance cast on them (for EW and FNP(4+)):
-On average, the Y’vahra with two Phased Plasma Flamer torrent templates could probably get about 4 hits per template, so say 8 hits total. At S6 AP2, that’s about 5-6 wounds, which ignore armor and cover, but FP still goes, so that is about 2-3 unsaved wounds (or one dead Centurion).
-Next, Ionic Discharge Cannon at BS4 gets 3 shots, about 2 hits, likely 2 wounds, and probably no unsaved wounds after 2+ save and FNP rolls. Same goes for the 4 x Missile Pod shots from Missile Drones, since they are only S7 AP4.
-Finally, the two squads of 6 Praetorians gets 12 shots total, with about 8 hits; S5 vs T5 means 4 wounds, which goes down to about 1-2 wounds after Cover saves and FNP… now you are up to an average of 4 total unsaved wounds ont he Centurions, or 2 dead models.