I would make a series of full 24 man trials. The first of the series would have Garuda, Titan and Ifirit all in one go.

The arena would have the pillars from Garudas battle, but Titan and Ifirit can also damage them, Titan would also gradually make the arena smaller as per usual. The trick would be to coordinate the damage on the primals so that you kill one first (probably Garuda to get rid of the pillar requirement before the other two can destroy them) while keeping an eye on the other Primals. (ifirit and Titans plumes, land slide and Titan jumping into the middle of the arena)

The second in the series would be in the water with leviathan, shiva, and ramuh. Levy would toss water puddles onto the arena that the other two would affect. Ramuh would electrify the gradually growing puddles and shiva would freeze the puddle and anyone inside. leviathan would keep his boat slam mechanic and the trick here would be to avoid the puddles as you slide. Would probable have to kill leviathan first, to prevent the entire arena from getting engulfed in his puddles.