Everything is usable anywhere. But some things are plain poop when not under the right field conditions.
Mega Swampert should be played with rain, this is my opinion. Great teammates are Politoed for the auto-rain, albeit for 8 turns (damp rock please), Kingdra which covers grass and hits special while abusing rain like MSwampert and Ludicolo which is very similar to Kingdra although adding grass STAB and Leech Seed to the team. Electric types are great too, for perfect accuracy of Thunder, which makes Thundurus a good teammate.