on any army and it is worth excising the problem. I know that the Exocrine seems more dangerous but with its limited range it can be held off with good maneuvering and deployment. This formation will be a job for the fast or the long ranged. There are no 2+ armour saves in these units so krak missiles are welcome allies in the purge. Plasma and bolters will do some good but you will have to use weight of numbers as plasma will not deliver instant kills and bolters allow an armour save.
If you like wave after wave of dross then this is for you. Three broods of Hormagants, two broods of Termagants and a brood of Warriors to keep them in line. This formation allows the gant broods to be recycled when they are wiped out. On a 4+ the unit is in ongoing reserves with all of the upgrades on options that were paid for. The unit continues to count as if it was the original allowing it to be continuously recycled.
These units give up victory points so killing them repeatedly is a good thing if you are playing for kill points. If you are having to deal with them in any other mission the first step is to kill the nearest synapse creature and then drop templates on the units to force them to break. A wave of units like that could seriously impede your ability to bring ranged fire to bare on the Tyranid swarms. I think this formation will be used to bulk out the numbers of gant units in a Tyranid army. If your opponent would be the type to go heavy on Tervigons and Genestealers then this is the perfect way to get some cheaper dross units between you and his more expensive monstrous creatures.