The fix is to fix the damn casting interrupt on taking physical damage.

Seriously, every other MMO out there that had this 'feature' eventually removed it because casters are just plain gimped in PvP when a single player can completely shut them down. The original reason casting would be interrupted in older MMOs was because magic was inherently more powerful (so had greater risk/reward ratio). Here casters do no more damage than anyone else so the interrupt design is completely arbitrary and unbalanced.

NINs would not be steamrolling casters so easily if they were actually able to fight back against physical DPS in the first place.