You can boost this with a second account. Log in with an Xbox Live Gamertag, guest accounts does not work! When you entered just stay in the back with the second player and be careful as the enemys are much stronger if you play with 2 players and only one actually fighting.
Krom is an easy one, as he isn't moving and shoots from his turret. To kill him, simply kill all enemies that you encounter on your way and clear the whole area, so that only he is left.
After that you may be lucky to stay next to a metal wall and have a small visual field through the peephole where you can use your sniper or pistol (or any weapon) with visor.
Be careful, Krom might shoot his turret to you and you get massive damage nevertheless.
When Krom dies you should get your achievement right away. You do NOT need to complete the quest, just kill the enemy.