Just because something is not 100% game-breaking doesn't mean it's not OP. That said, there are four keys to holding a base in TR territory. 1: Lots of air shields, these make it difficult for Prowlers to shell from long range because they tend to catch shots that have to arc a lot. 2: Lots of sources of repairs. Spam repair modules, fill the base with repair sundies, have lots of Engineers. 3: Shield modules, both to protect the HIVE and to keep Prowlers from shooting through windows. 4: Lots and lots of aircraft. Massive air superiority is your best bet for keeping Prowlers at bay.