I guess this is an exploit/bug since I've never explored a boss before where this was possible.
Aye, as title says, you are available as a hunter to shoot/fire your gun/xbow/bow while standing in melee range/under on Valiona and Theralion. Ofcourse, I'm speaking of the one standing on the ground.
So yeah, basically you can stand with the melees, do your thing, and spare yourself for the running when Valiona does blackout e.g.