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Yeah, my team is pretty weak to mega mawile as well. I do carry a will-o-wisp for it but that move always seem to miss when I need it the most. Even so I have manage to out smart people using it and win but I have also lost to it due to how powerful it can be. In my case I've never really had...
It is pretty frail even with the good typing it can be taken out in one hit by most physical attacker or special attacker that hit it super effectively.
It Hp 57 Def 91 S Def 87. Sure it has descent are defenses but Hp really bad. It more or less if you get to hit by yourself the main thing that...
I'm in favor of not banning it. There are a few things that you can do to prevent it like taunt, safeguard, misty terrrain, magic bounce and using a lum berry on a pokemon to get the boost and power and knock it out in one hit could work. Sure the strategy is annoying and a pain to deal with...