Dawn of War: 3rd Generation (DE) 1v1 Necrons vs Chaos (Insane AI)

While not the most amazing tactic, a good strategy to counter most races is to mass Necron Warriors while using your Necron Lord (and a sacrificial squad) to tie-up any melee enemies. Though you have to be careful when the enemy starts producing vehicles as you'll start losing units faster than you kill enemy ones if you're not careful, hence why I use the Necron Lord to harass them away from the rest of the army so they can focus down production/resource buildings.