Clive Barkers Jericho

On the level called "Flesh", Shoot the bodies on the wall so that instead of the behemoth spawning, three cultists spawn instead.

Just keep your distance, kill the three with Black's ghost bullet and another three will spawn.
You can get the full 250 kills here relatively quickly compared to the other levels.

Took me around 40 minutes.

On the Level Flesh, in the last room. you have 2 demons and four bodies on rollers. kill the bodies on the rollers and the demon hanging from the ceiling will come out and show his back. Kill him when he does this but don't shoot the other demon on the Pedestal. that guy will then raise 3 cultist from the blood continually, giving you a never ending supply of targets to work on your kill achievements

I did this and Stood over to one side of the room so that the 3 cultist came up in almost a straight row the minute they came up I fired the Ghost Bullet and guided it through all 3 of their heads which helped to work toward the "pop the heads" achievements while I waited for the Ghost Bullet to recharge I used the grenade launcher to work on the "Disintegrate" Acheivements

I used Jodmeister's spot for getting "God of Pop" achievement. As you're going for the headshots, you're also double stacking on ghost bullet kills. Here's his solution, credit goes to him I got 110 ghost bullet/head shots in about 50 mins.