Counter-Necron

9 of the top 16 lists at Adepticon included this:

Lord,
3x Warriors + Night Scythe
3x Annihilation Barge

and 8 included at least 2 6-man squads of Canoptek Wraiths w/whip coils. These are purely to tie up and live through dedicated CC and heavy weapons units. They are also good at popping AV10 vehicles. They're a really good jump infantry unit because they ignore everything but impassable terrain so they can fly in a straight line towards whomever they wish to engage. They also have 3+/3++ and are thus very hard to kill even with just T4 and 2 Wounds. Thankfully they don't have power weapons. It'd be grounds to ban Necrons from tournies that is, combined with Night Scythes and Annihilation Barges. People think Heldrake is under-costed. Look at Annihilation Barge! 90 points! Heldrake should be 80 at that point-scale.

Anyways, ranting about poorly costed units aside how do you counter units like this? Because, to be honest, these guys can ally with damn near anyone, cost nothing and are going to be Top 16 of every competitive tournament so building lists purely to counter these will probably get you far. 

Tau as Allies

Pretty simple idea: how to make Tau into an Ally force?

It's actually harder than it looks. Unless you're using 1999+1 you're going to be in trouble. Their troops are bad enough that no matter what you do with them you'll be wasting points. And, if you want to buff them, you have to get an Ethereal or a Fireblade which hurts the Ally slot because you can't take Signature Systems like a Commander can. The rest of the army has a lot of trouble standing on it's own so now you've got a much harder question than at the beginning.