The relevant superweapon is probably ADHAÏC CALYPSE, which is likely to be trotted out as an effective counter once a Tyranid hive fleet hoves into view.
(For anyone who doesn’t remember ADHAÏC CALYPSE, that’s the swarm fleet of self-replicating autonomous war mechanicals with fast-burn capability and self-improving mesh-networked warminds - which in its ability to use entire planets, not just the biosphere, as swarm raws is one of the few weapons systems capable of out-Tyranidding the Tyranids.)
Of course, even if it works against Hive Fleets heading to other people’s worlds, this is likely to be a controversial strategic choice:
“You unleashed a self-replicating horde of planet-eating abominable intelligences on the galaxy!? Are you INSANE!?”
“…it was Tuesday.”
(On another controversial strategic choice, no-one’s going to like the inevitable response to learning about Orkoid life’s spore-based reproductive strategy being “nanomachines, son”.
But, hell, that’s not even a military response. That’s just what the Ministry of Public Health does when confronted with icky, icky pathogens in their nice clean ecosphere.)