The description of this Trophy is deceiving as it may not pertain to just soldier enemies (generic enemies). I replayed stages over and over again and used Luffy's style action (triggered his haki) to stun numerous enemies, but I believe this Trophy requires you to stun boss.leader-type enemies. To do this, they must do an attack that causes a yellow jagged line to appear above them. when that happens you hit them until they drop to their knees and stars spin around their heads - they're now stunned.
If I got this wrong, please let me know so I can rectify this solution.