Now that's an interesting plot twist and explains why the MC was capable to kill with cold blood when he was first summoned, he wasn't training him just to be able to fight but to actually survive in that hell...
well considering the plot armor they're all fucked and it's all because they gave him a platform to exercise power eventhough they did it to "cage" him. stupid ass sides overestimating their power not realizing nothing could win against plot armor