Clive Barkers Jericho

If you are like me and didn't use the cheats for infinite ammo, or just couldn't acquire the cheats then this is the route I would take.

Start up the level "Spiritual guide" you spawn as Black, *The one you need to use for this achievement.* You are alone.. so you don't have to worry about your squad mates stealing your kills..

You take a pretty straight forward route through this level encountering quite a few Cultists. Use your LT attack to blow each of them up, and when you run out of ammo, simply let one kill you and start over again.

Now I say let one kill you because if you die your kills count, but if you were to just stop and reload checkpoint YOUR KILLS DO NOT COUNT. *Tried and tested*

By the time I hit this level I already had well over 100 kills. so I simply did what I stated above and popped my achievement in no time.

The above guides all work, but this beats playing an entire level over and over again in my book. You're guaranteed kills this way and you don't have to worry about your teammates getting in your way.

I recommend to use Black's grenade launcher (LT). Play the levels "Blackwatch"´, "Ambush" and "Flames Of Anger". You are getting ammo after each checkpoint - more ammo, more kills! After finishing "Flames Of Anger" just replay these levels until you get the 250 kills.
You can use your sniper-sight for this too!

Simply use Black's grenade launcher thats attached to her sniper rifle for the quickest easiest kills. They only count when they explode into bits and even if you die the kills will still count.

If you don't have this by endgame just reload any level, disentigrate all the enemies, then reload. Rinse and repeat.