How about just making the server do double and triple checks of player position when they're doing aoes? Most of the complaints are based on titan and he's pattern based. Seems like almost all the bosses are, anyway. Test the game based on 300-350 latency and see where the cast bar and actual attack animation differentiate and do checks on both of that. I've seen fighting games with training mode that lets you set pretend latency to let you practice the game as if you're fighting an opponent with high latency.

Or stop making boss fights based on aoe near-if-not instant kills if you're so scared of harming your cheap servers, SE. I'm sure there are other ways to compensate for the leniency.