Mega Pokemon are far from OP. As long as you are well prepared you can easily deal with most of them. At one point someone sent out a Gengar against my Tyrantium. I figured that Tyrantium could take on a regular one but I didn't want to take my chances with it's mega evo so I switched to snorlax. Sure enough he used a mega stone before launching a shadow ball at me, but snorlax's normal typing let it escape unscathed. Had I not switched tyrantium would have likely been dead.
can only use one of them and a good type advantage or defensive play can usually wreck them. there are only a few that I see having potential to become major threats
Garchomp: hillariously walled by togekiss, loss of speed makes it easy to cripple with status moves,
Gengar: loses levitate making it weak to ground moves, pretty much anything that outspeeds it can demolish it's nonexistant defenses
Lucario: loses flinch resisting abilities and is pretty slow without a scarf, anything with a good fire type move can make short work of it
Obamasnow: still has alot of not very friendly looking type weaknesses.
Blaziken: either haze away it's stat boosts or cripple it with status.
And I think that's enough.