Load up the level FRONTLINE
at the start of chapter four. Make sure you have a scoped weapon to make this easier for yourself. You can load up the level right before this one called ARMORY
where you are able to buy and customize weapons. I recommend the Hellbreath with a scope because you need 30 kills with that weapon for
Also make sure to stock up on both normal and fire grenades for
Now back to FRONTLINE
. At the start of the level you will see a train cart with soldiers on it(the one you fell off from). The commander is giving a short speech. Shoot him in the head. When he is dead, reload checkpoint. Reloading checkpoint does not reset your progress towards any of the cumulative achievements so this is a great spot for farming most "Kill X amount with X weapon" achievements.