On the 2nd fight against Zagi - the one on the ship - you need to knock him overboard into the water to "cool him off".
Setting this up can be a pain, but persevere and you should be able to back Zagi up against the side of the ship. When he's there your best bet is to hit him with a Fang Strike (???) to get him into the air, and then a Crushing Eagle (????) to knock him over the edge. If successful you'll get a mini-cinematic of him dropping into the ocean, and 'Great!' will come up on screen.
The trophy will pop at the end of the fight, so you'll still need to win!
N.B. You may want to put everyone else on defend before the fight starts. I didn't, I just turned off all their special artes so they wouldn't kill him too quick. Probably depends on how leveled you are at this point.