User blog comment:B214/Volume 22 spoilers/@comment-207.96.205.3-20161013163608/@comment-9595671-20161013195757

Yeah, use the Evil Dragons for an example. Grendel and Nidhoggr are considered barbaric while Crom Cruach, Azi Dahaka and Apophis are much more civil and honorable. I think it's this trait of their's that makes them stand at the top of the Evil Dragons. So while Trihexa may be more powerful in a physical sense, one's strength has to be measured in both body and mind. Rias did mention why intelligence mattered less than strength she told Issei to still get strong in body AND mind.

So since Trihexa doesn't seem to be intelligent in any manner, while it may be stronger physically, Great Red and Ophis can both think which lets them plan and even make new techniques while Trihexa's probably stuck with all the powers it was born with.

Plus... if Rossweisse's sealing technique could have prevented it from ejecting it's core, or it never had that core in the first place, the fight with Trihexa could have possibly ended right there. Still, the fact that to destroy Trihexa requires both GR and Ophis means one on one, they'd be too evenly matched, or the damage it would cause would make stopping Trihexa a pyrrhic victory.