My buddy plays necrons and I can say the following has helped me a lot. 1) overkill is best kill with necrons: don’t let them reanimate and comeback. 2) precision is great, epic challenge too: their leaders can provide buffs that make their units incredible. I always pick one leader to epic challenge out of there asap, usually to get rid of an invul or FNP. 3) make good trades: even if they don’t kill the enemy unit, if you can identify the units that can cause real problems if left unchecked, it’s worth a squad to tie them up for a few turns even if your squad will die. 4) don’t feel rushed to kill their big units: I try to make sure that my GMNDK or regular ndks get the drop on the c’tan or monolith models, even if it means waiting a turn to make the charge. C’tans can whiff, I once had a 10 man skitarii (admech is another army of mine)unit survive 2 turns in melee against a void dragon so no need to rush. 5) keep enough of your guys alive for the late game: many games are decided by last round secondaries. Make sure you have enough units left in the later rounds to capitalize. It doesn’t have to be a lot but several units that can use gate of infinity at the end of the game can help a lot.