When you fight Zagi in the tournament at Nordipolica (the third boss fight against him) you need to use magic and allow Zagi to absorb it. If you only use physical attacks it's not gonna happen, so take Rita and Estelle with you.
I did it on my second playthrough, so put Rita and Estelle on minimum damage and magic artes only and he destroyed his blastia very quickly.
If you haven't got those attributes, give them weak weapons, put your fourth character on minimum damage, and just evade Zagi. He'll break before they beat him. You'll get the mini animation and 'Great!' will come up on screen so you'll know when you've been successful. Then you can go all out on him.
As with all the secret missions, the trophy will unlock after the fight, so you'll still need to beat him.