Casket of Fury

Initially, I used Sleen, and as mentioned before, there was an issue with him. I only used Zerok when Sleen was unavailable. Later, I mainly used Lina's spells, and Sleen's Acid Cloud (much better than his other ability in the later stages). When the Reaper appeared, I used Lina's Gismo and the Reaper. I preferred Black Hole and Fury Gathering for him; if there was a lot of Fury, and they couldn't move, I would use Acid Cloud.

P.S. Sleen's ability is very good for fighting artifact guardians.