The trio's best approach would probably be to have Luslec far away supporting with spells while Gustang and Traumerei act as the main attackers with 3D Disconnection and Blossom's flames + Okbi + Library summonings (the main threats that could potentially deal big damage to Urek I guess).
Urek's definitely going to put a lot more effort into this than what we've seen on-screen. Part of the reason he looked so overwhelming at the Sprout was because the FHs had almost completely exhausted their strength from all the fighting. Gustang speculated that Urek wouldn't be able to deal with 3D Disconnections indefinitely and that if the technique had been fully powered, he wouldn't have been able to break it so recklessly.
Still, if Urek's fists are comparable to Blossom's full flame, he shouldn't have major problems as long as he actually bothers to use his brain and proper skills. He's also comfortably the fastest here, and his attacks have more than enough potential to one-shot if they land directly. The trio is going to have a hard time defending against that power and avoiding the hits.
While on paper the team is broken, I still can't imagine Urek losing this. He has only shown relatively basic moves for the most part, and those are already overwhelming. Granted, I don't think we've seen any of the other 3 at full power either. Luslec wasn't fighting properly against Urek, and still had a lot left before stopping the fight. I suspect that Traumerei was missing a considerable amount of power in Leviathan + didn't consume all the Shinheuh he gave to his BHs either. As for Gustang, he was said to have the abilities of all his Head Librarians but only used Tiara and Richie's (coincidentally, the only ones confirmed as dead/having returned to shinsu) + Traumerei said he was toying/messing around with him prior to going full power.
We're still in the dark about a lot of things, but as of now I'm betting on Urek with High-Extreme Diff.
