Frog boss. He goes to center of arena, and water puddles appear. He does six big wave / splash casts with new puddles appearing each time.
The puddles get a rough-splashy appearance after each wave, and explode with a big AOE with the next wave hits.
Stand in the NEW / smooth puddles to avoid damage (hit sprint at start) If you get hit, you get tossed into the air long enough that it's nearly impossible to reach another in time. If you can't get to a smooth one right away, just stand in the open. The damage is much less than from the exploding rough puddles.
Do not run near the rough puddles. It's better to stand in the open and get hit with the wave than by the explosions while trying to get to a smooth puddle, unless you have one right next to you and not near any rough ones.
Pic for explosion sizes:
https://i.imgur.com/w3m85b9.png