Take some advice. Roll a Merc, learn how to play the class properly, not just face tank like noobs do. Then you'll be able to anticipate what they are going to do and know how to counter them.
If you are getting smashed by Mercs, then maybe they are just better players and it wouldn't matter what class they were on.
Any over tuned class, wether it's a small or large over tuned one, will seem extremely OP in the hands of people who really know how to play the class and are good.
Most of the really good Mercs at the moment are ones that learned to play them properly before 5.0 and probably main them.
So I suggest you look inwards to discover the real reason you think they are so OP as to ruin the game. I'm pretty sure it's a L2 improve your skill level issue.