How to remove mirror stickers from a painted wall?
Sounds like you're dealing with a pressure sensitive vinyl. It's not going to go down with out a fight. The best course of action is to hit it with a heat gun and very slowly pull with light steady pressure, let the heat do the work. If you do it right the adhesive will detach from the wall and the vinyl in about equal measure, which of course means you'll have to deal with the residual next. If it's the kind of adhesive I think it is, Goo Gone works okay (or any of the orange oil based cleaners) but Rapid Remover is designed for the job. If none of those will touch it then it's something solvent based and you'll have to step up to something with acetone in it (Goof Off) but test a little spot first to make sure it's not going to ruin your paint. He's a link with a bit more information.  http://www.signwarehouse.com/blog/a-basic-guide-to-vinyl-removal-options/