I just did this quest on my Witchdoctor last night (I think she was level 14) and I saw that the goal was to defeat Nim, the rat. I sent my two companions (Battacuda and Ratbeard) on a direct assault and got him down a round after they had both died from the other enemies attacking them. Bummer, but at least I finished it, right? Nope. The battle continued and I saw that the victory condition was now "Defeat All Enemies". Uh-oh. There were 3 left against only my Witchdoctor! Time for some hail-mary strategy.
I was able to knock out one of them with an ability, but the other two got some hits in. At that point, I used hide in the shadows (have that card on my hat). The remaining enemies, one at half health, the other at full health, ran for the south of the battle board. I ran for the north and healed myself, which brought me out of stealth. Then, I cast slow on the full health mob and tried to burn down the half-health mob before he reached me. I knocked him out, but not before he got a few hits in. Now, the full health enemy starts making her way up the battle board and I'm at half health with no abilities left. I start hitting her with my wand and she's at half health when she reaches me. She gets a critical. I get a regular attack. She gets a regular attack. Now I'm at 2 health and she's at 120ish. I get a regular attack. She misses. I get a regular attack, she moves and doesn't attack. I get a regular attack and she's knocked out.
I got incredibly luck on those last two hits (or the game had mercy on me) but I wonder if it was a glitch that the battle conditions changed the way they did. Obviously, I didn't remember this particular battle from my first runthrough, but I do make sure to read the conditions carefully, just in case. In this case, that backfired.