hrm. tough fight tbh. i guess IB could always resort to the whole--pull out the opposite trick he's used. not 100% sure he could do that to someone on the level of trigon though. there is quite a bit of narration that indicates trigon is a universal level threat, but little on panel would support that. given that galactus (classic) was able to hold his own against IB, i wouldn't say the power difference here is much different. personally i'd say trigon is in roughly the same tier as classic galactus. on hype, i'd say trigon would take this, but on feats it's tough to say for sure.
2 characters who are both a bit tough to gauge and don't have too many appearances makes for a tough match to call. i do think they are close though, so not a mismatch by any stretch.