For this you must utilize the shotgun's shrapnel mode. Use it to indirectly kill 10 enemies. Best done in confined areas where enemies cannot move away. Remember, reloading checkpoints work in this game. You get the upgrade in Chapter 11. Once you kill 2 shotgun troopers there are 5 or so sailors in a corridor. Simply shoot the walls and watch the projectiles bounce around killing them easily. You only have to get 10 kills so it is not too difficult.