This trophy is missable.
In chapter 10, Tachibana and Oda will go to Tojo HQ with you. Prior to going, stock your inventory full of items to restore heat, like Staminan Royales from the pharmacy. During the fighting portions in Tojo HQ, you meet up with Tachibana and Oda and they fight alongside you after the first room. Wait around for them to grapple with enemies. Once they are grappling with someone, you can run up and push triangle to perform a co-op attack as long as you have enough heat. Since you will lose your heat waiting for them to grab people, throw some punches once in a while, and drink your potions as needed to maintain high heat. It should not actually take very long to get the trophy if you just dodge around and let the enemies swarm your allies. They do the grapple very frequently if you let them. The trophy should pop the moment you get your third kill. Bear in mind that this will only count if the enemy dies from it, so set the difficulty down and use your strongest fighting stance if that becomes an issue.