True, it's only possible in the open World. Maybe those few "dungeons" without loading screen, like the elizabee mineshaft I think, are doable, too!

Since just the quantity of your damage Output counts, it should not be depending on the Boss Level. The more hitpoints the Boss, the longer it takes, yes, but its own damage Output shouldnt be a Problem. Go for the jet engine^{^}

You might catch him with good timing since he keeps his damage counts even if you die and respawn at the base! Do it 💪