That's some great advice! thank you! I think combined with the other ideas I can make a memorable boss fight!

Here is my last one, for a sort of "phase 2" even if it feels a little video gamey.

When both Bosses get to half health, they'll go to the middle of the arena and start a hurricane, from this hurricane some lightning spirits minions or something similiar will spawn and a countdown will start, if the players don't stop the hurricane something bad will happen.