How do i remove a mirror that has been stuck on with Mastyk (sic?)?

If you do not have a small hand saw, slip a butcher knife under the mirror and "saw" the mastic. Work in sections and if the mirror is big have someone handy to help. As you release the upper right section, move to lower right. Then move to the left lower and then the left upper. Have someone hold the mirror and gently open on side so you can get to the mastic in the middle. Do not open wide or the pressure make break the mirror. If there's no mastic in the middle, you're in luck and it should come off. If you've damaged the wall, use spackling to repair. Goof off to remove remaining mastic. Good luck.

Wear gloves so you don't get cut. Heat. Hair dryer will be fine. Heat it for a while then put a small stick in wherever you can and gently start prying up until it starts to move. Keep applying heat and pulling gently until all of the mastic releases its evil hold.

saw a post on Pinterest (I think) that showed the mirror being taped with duct tape first to avoid shattering then a putty knife was used to pry the mirror off the wall