That's something which took me a while to figure out too.
It's not exactly squares, but more that when the red spread marker resolves, your character also drops a cross-shaped "puddle" below them. Since team members tend to cluster together anyway, these crosses often intersect to form squares.
Personally I just make sure I move away from my own cross, while making sure not to go too near to anyone else's cross.
My own problems with the bosses in the dungeon is on the first one, and it's just something that I can practice for.
Namely, the hitboxes on those brooms are a lot bigger than I expected. Even worse since they knockback.
