Complex sentence: Very few planets manage to fend off a Tyranid invasion without resorting to Exterminatus (wiping out ALL life on a world) and even those that do spend years on "mop up" work of rooting out the last surviving Tyranids.
Split into simpler sentences: Very few planets manage to fend off a Tyranid invasion without resorting to Exterminatus, wiping out all life on the world. Those that do spend years on "mop up" work of rooting out the last surviving Tyranids.