In general, "the" trick (if there's only one) is to use garrison, teleport, and tunneling to get your opponent in a very difficult position on turn 2 (or 3). In other words, give your opponent more assaults than he can handle even after winning strategy and retaining. Genestealers, lictors, and trygons can do a lot of heavy lifting in this regard. You have to accept that some of these are going to be sacrificed (because 1-3 might be toast by the time you activate depending on skill and placement on the board), and have equally good alternatives waiting in the wings. Genestealers and lictors just got schwacked? No worries, there's 2-3 big assault swarms that the enemy wasn't able to touch... You have to use cover, prep assaults with BMs and support, etc.
As for ranged firepower, don't underestimate the resilience of the dactylis, and the disrupt ability on those and the biovores. Additionally, you have exocrines. The zoanthropes are there mostly to make it difficult for air to return (because of BMs). Maybe this stuff doesn't get many shots off, but in that case, your opponent has most likely allowed your assault swarms to move up unmolested.
This takes practice to accomplish successfully though. Build high-activation count armies, otherwise a shooty army will take the 'nids apart piecemeal.
EDIT: 1000 posts, yay!! Ugh, and it was on a 'nid thread.
