A flat damage decrease won’t really help. IMO it would be best if the game handled it the same way Dragon Nest did: each skill has both a PVE and PVP effect. Skills are quite varied and slapping on damage decreases across the board won’t make PVP fairer. Some classes will also suffer because other classes retain the arguably “unfair” skills that make them OP in PVP.
Dragon Nest doesn’t do this (insofar as I remember) but it’d be nice if you could switch between a PVE and PVP skill set-up.