Fire Belial Solo

Pretty straightforward, just heal as needed and save Mika 1 to use on Chichiri if the boss actually manages to build up any diamonds. You also want to save Percy 2 for post 25 and use Chichiri 3 early enough that you have it off CD for 5-0 to give yourself extra wiggle room with damage (not that you probably need it, I killed with plenty of damage to spare in one turn).