Ancient Amuletor (HP)

This trophy requires you to use the archer. First get several kills with the archer and unlock your skill. Then once the skill is available allow 10 or more enemies to amass, hit the button on your left controller, draw your bow then aim at each enemy until they are highlighted with a circle on them. Once you have locked on to 10 enemies release your shot. I would avoid bombers and shield soldiers for this. You can actually get this during the training if you wait on the third wave for all of the enemies to spawn before you attack.