I'd recommend, if possible building up, rather than tearing down.
Check with the manufacturer of the thinset for buildup. If this won't work for aesthetics or other reasons, get a good cold chisel with a wide end (about 1-2") and a hand protector and have at it. A brick chisel might work well for this. Make sure and get a hand protector though, it'll be worth any and i mean any extra money.