You need to make sure that you can throw enough shots down the table at all three ranges adding on more and more shots as your opponent gets closer.
36” + At this range we have destroyers and heavy destroyers targeting transports, vehicles, and monstrous creatures. Their goal at the start of the game is to slow down that mech or running monstrous creatures.
24” + Mid range is where the immortals kick in now targeting infantry with their S5 shots along with the monolith particle whip.
12” + Short range has warriors rapid firing, flux projectors on the monolith, and tomb spyder staff of light shots, etc.
A good Necron army will have shots at each range so no matter where the opponent is on the table they are throwing out something to them…