HELP Asag keeps respawning

I'm doing the quest on the moon, the one where you follow Columbina, and I'm fighting Asag, but I can't get it below 75% health cause it keeps respawning with full health (it's not distance based, I even tried making sure it didn't move from it's spawn point by only using the sword)... I've nearly finished my healing food to keep trying and I even tried to use some of the buffing consumables I had hoping to kill it in time, but to no avail, what should I do?