I have an ACTUAL solution to this:
1) Hit the monster you can reach (not the goo/slime, obviously) with your spell when you're to its NE (up and to the right of it), so you hit it down and to the left. Then use the spell to hit it from directly above, knocking it directly downward. Now you'll have 25 Piety.
2) Use your Piety two buy Stone Heart and Stone Skin. This will reduce the slime's resistance to physical damage to 45% and increase your damage resistance. The side effect of buying the boons removes stone tiles, exposing the slime to you.
3) Physically hit the slime once, then physically hit the other monster and kill it, resetting your HP and MP.
4) Now you can hit the slime twice physically, and knock the slime through walls twice with your spell. Be sure to attack it from angles that will knock it through stone tiles, doing additional damage. This will let you BARELY win. Congrats!