The way to kill Slezz is easy if you know what you're doing. After that nice cutscene go onto the platform with the lever. Pull the lever and let her burn!! It's good to stay in the middle of the platform and as close to the edge as possible. While the acid is burning her skin she'll throw suicide bombers. Wait until they're close enough then harpoon that lil' sucker and fling him towards Slezz. Keep doing that until Severis says you can cut her heart out. To do that just harpoon it. Oh yeh one more thing, watch out for the acid she shoots from her...ummm...well just watch out for the acid
and the minions.