I have completed it once before:
https://www.youtube.com/watch?v=g30UsQA8bTEIt takes quite a while of strategic planning, but it can be done. There's also a tiny bit of luck involved in where each of the AI move to (for example, the high level pyro moving around to the left in my video gave me a fair amount of time to deal with the right side before turning my attention back to them).
It should also be noted that this was before an apparent bug was fixed, where if a character attacked and leveled up, they weren't supposed to get an additional attack.
I tried the battle once after the extra leveling attacks bug had been fixed but didn't succeed. I also had the high level pyro stay on the right side that time too, so it was a bit harder in that regard too. Not sure if it's still possible to kill everything without farming exp in the previous couple battles, but maybe. I still think the best strategy is holding up at the bridge and trying to separate the enemy forces to go on both the right and left sides, and utilizing the water, and Motivation as best you can.