This can be really annoying! I recommend playing the mission on easy just to complete it and unlock modifiers. Replay the level on any difficulty with modifiers increasing your damage and health. I used Duke for this achievement. Storm shadow appears three times to try and steal the fragment. First time he runs at you as your coming down the temple steps. He goes down quick here. Second is in the long pathway after the waves of enemies. He runs straight at you and goes down with a few shots. The problem is the third time when he spawns close to you after going down some steps. Don’t bother with an ultimate because it freezes you in place, just keep back pedaling and shooting and don’t get caught on the walls.