Well, it was most probably because Isles did not have {Demonic Build} to recover his HP. The players had not lost all hope yet. Well if the boss could recover 1% HP once in every 30 seconds, they players should just kneel down and give up on the boss battle.
The system must have predicted that only a few would be able to survive until the end of the tenth wave. So, they designed the boss to be less threatening than he should have been.
"Mortals, you have only one way out of this! Die before the great demon! Give up on your pointless struggles and become my slaves! Then, I'll let you live!" Isles let out a series of strange and bizarre laughs. Then, he waved his axe and summoned countless meteors upon the players. They were bombarded so violently that they were jumping around up and down in an attempt to dodge the impacts of the meteors.