Easy way to do this is by pressing
+
when the boss is low on health. This will command the entire attack team to stop attacking and to fall back. Once they do, the kill is yours. If it for some reason doesn't show you as the last attack, you can hit the home button and close the game and then relaunch it again for another go. Don't worry if you don't get MVP because that doesn't matter towards any trophies.
Note: You must keep the last attack bonus item and must not sell it at any point until you get the trophy.