@Scooby Doo
And those Cat 1-2 will be quite handily dealt with by regular 40K firepower. Despite what you think, it's vastly more powerful than modern day weaponry.
One single weapon destroys any Kaiju. Warp bomb = Dead Kaiju. They have no defense against it, and since they have no understanding of the Warp...no way to develop a defense.
@Scooby Doo
Apologies, I didn't mention the Ripper Swarms main tactic is to just dig themselves to their targets and erupt nearby them. Heck, they could just get into the Arachnids' hives that way as well. So...subterranean rumble ensues.
@Scooby Doo
All the 'nids need to defeat the Arachnids is a large enough Ripper Swarm. They'll consume everything the Arachnids throw at them with no need to build anything bigger. Biomass now secure. The Arachnids are actually vulnerable to a swarm that's more numerous than theirs.