I'm sure Moose will correct me, but to me it looks like this:

Shinryu when summoned would have wrecked us pretty badly. But Omega fought him, beat him up, and even locked him in a prison that might have weakened him.
Zenos has trained in battle since he was a kid and even gave himself fake echo. That why we couldn't just faceroll him like everyone else. But once he merged with Shinryu (the greatly weakened Shinryu) we could use the full power of the Blessing of Hydealin against him and actually win. Of course he was still very much alive after that, but since he got the epic battle he wanted he killed himself now that his life is complete.