You can rearrange your party formation such that just when you are about to trigger the fight by reaching the sword, at least 1 character is not in the pool with the rest. Takes a bit of trial and error to figure out the exact point of approach, but it ensures that at least 1 character (preferably a melee) does not waste 2 turns climbing out of the pool and getting to him.
Oh speaking of this, here's another funny thing I noticed yesterday when it comes to cheesing the party formation. Suffice to say, Pierre's one step ahead of you here as well.
Remember the shadow clerics in the death snare tomb, who always summon ghouls on their first turn? Well, one of them has improved initiative - the one who can actually end up within reach of your wammos upon fight start. This French asshole has given improved initiative only to this one enemy in the whole fight just to make it harder for you to stop him from summoning the ghoul, I mean FFS