User blog comment:Trihexa/Top 10 Most Powerful Beings In The World (Unofficial)/@comment-26911440-20160527210356/@comment-24.113.252.213-20160528000402

It was said in the Light Novel, by Shiva himself, that "only Ophis in her full form and Great Red can destroy Trihexa." That means that either one of them can do it, not that they both need to team up.

Also, I think it's more that Ophis could force Great Red to leave the Dimensional Gap, but doesn't want to run the risk of being killed or at least severely injured. She gathered the Khaos Brigade to just increase her chances of winning.