Der-Suul was the only tough fight in the entire game for me. The only boss who made me start scraping for survival, chaining resurrecting items like crazy. I did beat him first try, but it was close, and I was like level 57. He's an infamous difficulty spike.
Here's the best tip for that fight: start with a high rush gauge, save it until he drops to ~50% HP, then try to one-shot him. Even if you don't quite manage, you can squeeze extra damage out while resurrecting your guys.
Spacing all of your ranged characters far away from his is good. Controlling Miki is also one way to do it. His explosion
is fire damage, so if you stack enough fire resistance and HP, you can tank it.