Checked your save, and Perun just didn't have enough energy at that point.
This was actually meant to be a "flaw" in their design, making it harder for them to effectively fight any long battles, especially since you can meet them before the final depth when you aren't as powerful, and taking on three unique NPCs at once could be really overwhelming (especially when they were first introduced to the game!).
For the next release I'll be giving Perun in particular an Exp. Energy Well.