@otakubouzu,
@Closedbox: Maybe his skills aren't the run-of-the-mill versions, but advanced once the goddess oversaw. Or those skills are new to this world and the goddess didn't anticipate, that her minions don't have a resistance or that the effects would last for an entire hour/until dead. There is a number of explanations. And the goddess herself stated, that their skill system has been arranged in a way, they should be able to understand it. Nobody said, that the rest of the world follows VRPG-game-rules. This way the reason is a unbalanced "reality" and a severe misjudgement of that stupid bitch.
I imaging, that after he defeated the minotaur and his levels skyrocketed, his MP or SP reserves (whatever his skill uses) went up for ceaseless abuse, which is something else the goddess didn't see coming, too. I guess as this dungeon probably is around for millennia and no hero, what so mighty ever, ever escaped, she is confident in it.
Now only one question remains: Should he a) clear it out after stocking up supplies or b) preserve it to throw a weakened goddess herself into it later on?