Suggested by Sonic Bahamut is a powerful fighter that took may shots before going down. That said, Trigon is one of the more powerful DC villains and has taken on many opponents in his day as well. He should be able to overwhelm Bahamut with pure power here and also has more long range options. He can dodge Bahamut’s attacks far easier than Bahamut would be able to dodge Trigon’s and that will make a big difference. Trigon wins.