Send extra people to the corridor that is flooding since you will have extra NPC enemies pop up there.
This is not a good strat for that part. Send your longest distance runner down there on turn one. Make sure they have a bow. Free the chair lady first and shoot the lever to free the other. Then go to the door and wait just outside. Once those two get past the door you can just close the door using the outside door lever and you don’t have to deal with those Sahuagin at all. They can’t get through the door. You only have to spend one turn and one person in there. Those sahuagin are menaces on tactician which is why I just lock them up.