"Spoiler" Pestilence mech. question.

Sadly i was trying to "power up" the pestilence mech. and i make it jump to the maximum. I manage to clear everything but the problem is the boss spawned in a absolutely dark cave with no way to illuminate thing (Using a 2 hand staff , my only weapon) plus the boss regen HP . For base line would be doable easily tbh the boss is easy enough but the regen plus my 11 damage per hit makes thing really sad.

Ther's a way to reset the pestilence? a Timer? something?