the first thing i thought when i saw the enemies and Dungeon Core in that room was to defeat the Dungeon Master or destroy the Core first but since the mc who supposedly knows more than me didn't think of that i thought the core is indestructible and killing the Dungeon Master wouldn't affect the Greater Demon, killing the weakest one first has always been my priority when playing an RPG, i hope there's an explanation as to why they have to fight this much other than to show us how strong the Greater Demon is.