Call of House Hardabar (Grimrock 2 MOD). Difficulty: Hard.
This boss is boring because of how long it takes to kill him, I should have killed him with death bombs from the beginning. You can buy the "shield of the elements" in Malan Tael's store for only 50 copper coins, it gives you "resist all + 10" and "evasion +7", one of the best items you can buy in the store in this part of the game, unbeatable value for money.