Jitsu Squad - They chickened out Trophy Guide
To complete this trophy you need to play as Jazz, or have Jazz on a tag team, and locate the Chicken staff. Based off my experience I'd say it's an uncommon drop, one I saw roughly three times per run.
Once you have the staff, complete the stage like normal. When you get the boss critical, simply finish them with the default triangle attack to turn them into a chicken. You get a good number of shots, roughly 10, so you don't need to worry too much. That said, the charged attack will not turn the boss into a chicken.
If you run out of shots for any reason, dying will refill your charge. It's a good tactic if you just want this done.