Sorry for such a long note, but I am just trying to be thorough and give alternate possible solutions.
This achievement is pretty easy, it just takes time. Kills in both the Campaign and Arcade mode count towards this achievement.
Considering that you completed the story mode portion of this game, then you should already be up quite a bit of kills. If you're on the hunt for manuscript pages or anything else, you'll definitely end up killing a lot more Taken, unless you're just speeding through the game.
An easy way to really rack up the kills is to play Nightmare boards of Arcade 'Survive Till Dawn'. Heck, you might pop this achievement before you even unlock the Nightmare boards. Anyway, the enemies in this mode just keep coming, granting you the option of kill after kill after kill. There are no waves in this mode, thus saving you lots of time from hunting for enemies. Although this might not be your most safe option, you can tone it down to the non-nightmare boards if you like. I reccommend the 'Ghost Town' and 'Caves' Nightmare levels. I also reccomend having at least 30 manuscript pages for the Sawed-Off Shotgun unlock for the Caves level.
Tips: do not be afraid to drop a flare when picking up ammo or grabbing weapons, use flashbangs when you can, use your most powerful weapons first (e.g. hunting rifle, combat shotgun, etc.)
If you choose to do Ghost Town (Nightmare), which requires 15 of 30 stars, straight ahead from where you start is the ammo spawn, and to the right of that between two buildings is the SMG case. There is a stairwell above that SMG where you can camp, the enemies will funnel right in front of you. I recommend getting the revolver which is on the execution platform in the street, and the hunting rifle (req 15 pages) which is inside a shack in the brush area at the opposite end of where you start. The flare gun I believe is right beside the ammo spawn on a crate. There are tons of flares around the level, pick them up as you make your way around the level getting weapons, but do not focus on the supplies, grab them as you go. When you need ammo, drop a flare or flare gun shot at the base of the stairs and head around to the ammunition case. The only way you can lose your multiplier is if you get hit by a grenadier or you jump from the top of the stairs. Sometimes a grenadier might get lucky and hit you. Don't fret, just keep focusing on what is at the base of the stairs. Their grenades will usually roll down the stairs away from you or completely miss. This is where I got my 'Survivor' achievement, but you can get lots of kills here. If you come across Splitters make them fully multiply out to maximize kills and they also become much easier to kill. As for The Birdmen (bird morphing taken), just keep flashing your light on them to make them fly away. Trying to kill them will make things trickier.
On Caves (Nightmare), this is a little harder. You need 21 of 30 stars for this level. The enemies seem to come at you in large groups more frequently, and running for ammo is a bit more dangerous. Stick with your pistol until it is out of ammo while making your way to the opposite end where there is a ship ramp. Nearby the ramp is a Pump-Action Shotgun. Up at the top of the ramp is a flare and a Sawed-Off Shotgun. When your pistol runs out of ammo pick up the Sawed-Off. When you are completely out of ammo, drop off the back of the ramp and head left towards the ammo spawn. This is an alternate way to get your 'Survivor' achievement.
But remember, the whole point is to get kills, which this is easy to do because these solutions are in which enemies have a one way entry point, and you can rack up the kills with ease. Ammo and supplies are the only issue.
This is my first solution, and is pretty basic. This achievement is not hard, like I said it just takes time. Please be kind with feedback.
(Most recent edit: 2 June 2013)
1- Some grammar fixes, added a few more hints
2- Changed enemy name to it's proper name 'The Birdman', stated that kills in any mode count, added stars required for nightmare levels
3- Grammar fixes; tried to cut back on the amount of text, moved the edit history, moved tips