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At my FLGS we have a group of Necron players and another Nurgle player. Playing against them is fun, but it seems to me that both, particularly Necrons, are extremely powerful. What would be the best way to counter this with Tau?

What is your list ?

I generally use a Fireblade, two squads of FW (one with carbines in a Devilfish), two Crisis Teams (one with plasma rifles, fusion blasters and shield generators, the other with missile pods and burst cannons), a Broadside with HYMP snd a Seeker, and finally a Riptide with stim and ion accelerator.

Potentially I could use another six man FW team, drone squads, or a stealth team. I also have another Devilfish.

Is it nurgle daemons or nurgle marines? If it's daemons you should get some markerlights and ignore their cover. If not, get markerlights anyway.

Daemons.

Necrons have a 24" range on 95% of their ranged weapons, you just need to shoot 'em up from a distance, and especially if they run a Metal Tide(Lots of infantry/Vehicles) sit back and shoot with Strength 8 or higher. I play Necrons and my biggest fears of Tau is Riptides(Obviously), Markerlights, and anything with a S8 AP4(or better) weapon.

Daemons always have 5++, so you want volume of fire. Nurgle stuff comes with Shrouded, so they'll generally have better cover save than the 5++. Markerlights solve those.
Nurgle has at least T5, ignoring Plaguebearers and Nurglings. So get S7+ weaponry. Firewarriors, with their S5, are enough to deal with those 2 exceptions I mentioned (of course, with markerlights)

Necrons are mostly T4 and 4+ or 3+. Again, S7 AP4 will force them to use Reanimation Protocol (Decurion aside, it's just 5+). Plasma will deal with the T4/5 3+, and Deep Striking Crisis behind vehicles will ignore their Quantum Shielding completely.
I don't think Melta is that interesting against both armies

I'd suggest to don't mix weapons with Crisis. Each model should have 2 of the same weapon, as it makes the model much more point-efficient. Same rule goes to the entire unit, unless you put target locks.
Shield Generator isn't that hot on them.

Ignores cover is the bigest bane to nurgle stuff i never put armour on my daemon princes and rely heavily on the 2+ jink save and wiping squads when i get into combat so i hate seeing ignores cover stuff.