User blog comment:B214/Volume 23 discussion blog/@comment-97.120.253.155-20170520052839/@comment-10330822-20170520193459

Its true that nothing is undefeatable, but your combined power needs to be greater than the individual power of Trihexa. In other words, even if dozens of God-Class enemies gang up on Trihexa, they can't necessarily beat it if their combined power doesn't measure up. Numbers don't necessarily give you an advantage.

Its been months since the battle with Trihexa started, and they are still fighting, meaning simple isolating Trihexa and ganging up on it haven't been enough. The best the leaders seem to be doing is locking it in a stalemate.

Its true, Issei isn't the only person who can do something, but he is the hero of the story, and its foolish not to recognize that. He's defeated most of the powerful enemies, so him coming back when he achieves his "god buster" status and killing Trihexa once and for all isn't unthinkable. Don't be surprised when this happens, this is an obvious shonen trope that everyone should see coming when it happens.