Break the Monolith formations or concentrate fire to outright destroy them. No available portals is a baaaaad time for the Necrons. Be aware that, since they are fearless breaking Monolith formations mean they can move closer to you and suffer no ill efffects, allowing for a portal assault so start with portals that have already been used (if possible).
Also, try to have the BTS broken at the end of turn 2 if possible. If they fail to rally, you'll have BTS in the bag for turn 3 victory condition calculations.
Watch for the Monolith double followed up by a portal engage move, it's pretty much the standard tactic.
Also, if you're close on strategy, pray they don't get first activation.
