Many have talked about this before, but the new Necron codex has some tremendous potential for synergistic combinations.
Now, if you have, say, 3 squadrons of 3 Heavy Destroyers each, that means you get 9 shots a turn that will hit 97% of the time, that will wound 97% of the time, and that will ignore all armor saves due to AP2. So, outside of cover and Invul saves, they should be inflicting 8-9 wounds each turn, especially since their Gauss Cannons have a 36″ range.
Even against vehicles, the S9 AP2 Gauss Cannons are quite effective, especially given their high accuracy, and the Triarch Stalkers can supplement them with their ability to each fire Basically 2 Multimelta shots a turn. About the the only weakness this battle group of Heavy Destroyers and Triarch Stalkers have is against either hordes (which Necron units like Warrior phalanxes, Scarab Swarms, Flayed Ones, and Tomb Blades excell against) or against Flyers (which Night Scythes provide a solid counter to).