Problem: you are fighting an ooze.

Solution: For the love of Pelor, don't get into melee with it! Sure, you can power attack for full and hit it every time, but it's an ooze! It will probably dissolve your weapon, or it will split in half, or the weapon will stick to it. Then it will engulf you, and dissolve you/pull out your bones/crush you like that poor girl at the end of Akira (the movie).

Hang back; oozes are really slow. Pepper it with ranged attacks, ideally magical ones. A warlock can kite an ooze to death quite easily.