After each add phase, sylvannis and her crew are supposed to attack Krosus and then he will return to the platform, but unfortunately they cannot make it to the island and end up doing zero damage. I had to jump into the ‘lava’ and use ranged attacks against him until his health got low enough to return to the platform. This happened on each phase. I was able to get around it, but it should probably be looked into.
I got to this battle via chromie timewalk, if that is relevant.