I have used wire behind mirrors to separate the glue/mirror from the wall. I used picture hanging wire which has a little grit to it. You need to find a corner where you can get the wire behind and just start sawing away at the glue. However, be prepared for breakage and it suddenly letting go from the wall. Cover the entire mirror in tape so if it does break, it won't be as dangerous to you. Also, be very prepared because once you start sawing away, sometimes the glue just gives and the entire mirror will fall at once and mirrors are extremely heavy. I had this happen and almost got a concussion. However, I have removed two mirrors this way and neither has broken (although I almost couldn't say the same for my head). Wear goggles, work clothes and leather work gloves. Good luck!