Summarize this article:

This will only work on some types of mastic, usually in recent homes. It's worth trying first, as wetting the mastic can make other methods safer. You should notice the mastic loosening within 20 to 60 minutes.  Mix the hot water with vinegar or citrus degreaser for extra strength. Old, black mastic may contain asbestos. Keep it wet at all times during removal, to prevent dangerous dust. Once the mastic is wet, try to break it off with a hammer and chisel. If it's soft, scrape it off with a wide putty knife. Use a long-handled edge scraper on floors to save bending over. Some modern mastics or mastics containing linseed oil will soften when heated. However, these are also flammable. Wear heat-resistant gloves and use the heat gun for no more than a few seconds at each spot. Scrape up with a putty knife as you go.

Summary:
Soak the mastic in hot water. Chisel the mastic off. Use a heat gun cautiously.