User blog comment:B214/Volume 22 spoilers/@comment-9595671-20160720041832/@comment-9595671-20160720061134

Yeah, I feel the only real possible ways to actually kill Trihexa? Either they'd have to seal him away again, and THEN afterwards they'd have Great Red attack him while he's sealed, if Trihexa could act while sealed then I doubt he'd be able to fight back under this.

Although that requires getting Great Red out of the Dimensional Gap, which would be a problem unless temporary, or having the Dimension Lost somehow move Trihexa into the Dimensional Gap so Great Red can just do it there. Though I don't know if Georg will ever get the strength for that in his lifetime.

Either way, killing Trihexa likely won't happen without Great Red, but the method of how Great Red does is may be the biggest problem in how they could do this. So... yeah, there's a chance your right about it not being possible, but then again Trihexa could end up the final enemy of the series.

After all, he's the only being who can surpass Indra and Shiva right now aside from Great Red and the latter doesn't really seem to be a threat to the heroes. Plus nothing says for certain EX has decided every event, who knows, something coudl change, it may be an alternate timeline like the third anime season even.

Guess we'll have to wait and see ourselves.