Ever considered coming in the
other way?
As you may know, there are two entry points into the Slaver ship. I usually prefer to knock my way in from the other side, as opposed to bum-rushing in from the sewer side.
Summon several spiders and sic them on the slavers. Cast multiple Webs to hold the men while your spiders devour them. If you can cast Greater Malison as well, do so.
Stoneskin and Mirror Image will protect you from the traps.