User blog comment:B214/Volume 23 discussion blog/@comment-97.120.253.155-20170520052839/@comment-10330822-20170520063939

They aren't sealed forever, 3 of the 4 Satans along with a bunch of gods from other mythologies, create a special field that traps Trihexa and them causing them to fight a constant battle against the beast. However, the outside can still provide supplies and they can communicate with the outside.

During the battle, Rossweisse started sealing the heads of Trihexa, returning it to its state before Rizevim awakened it. However, Trihexa defensively shot out its "core" which then sought to fight Issei after it sensed the power of Great Red and Ophis coming from him. Issei continously burned Trihexa, but it just kept healing the damage.

He hasn't really written himself into a corner, he just hasn't developed a way yet for them to "actually" kill Trihexa. So, its like he hit "pause" on the final battle altogether.

It actually makes sense, because the Trihexa is essentially the strongest being in existance next to Great Red, and Issei and the others would need to be stronger than most beings in order to be able to help beat it. The "pause" situation is probably so Issei can eventually become stronger enough to kill Trihexa, and free the sealed rulers of the mythologies.