This has been edited to look & read better from original "big wall of text" lol.. Ok have fun doing it in campaign like everyone else suggested while you have a helluva time counting actual kills and respawning if you die ect... I suggest 2 player private deathmatch. Invite your friend and fire away! Set the game for 2 players max, unlimited time, and 100 kills to win on Rachel (it's the smallest map).
My buddy and I made a grid of all 6 weapons for 100,200,300, and 400 kills, checked them off as we went, never lose track, easiest/quickest way.
Pick the weapon your gonna use and stand next to it, everytime you kill your boosting partner, he runs back to you rinse & repeat. It takes about 1/2 hour per 100 kills with each weapon. Muliply that times 4 for 400 kills that's 2 hours per weapon multiplied by 6 weapons = 12 hrs for achievement (per person).
The best part about this is the other person doesn't even have to be in the game. Once they join they can walk away because after your 100 kills, the game will reload after a 10 second warm-up period. You will just have to run to them since they are idle, takes maybe 10 mins longer each game.
Quick side note, for Plasma rifle and Rocket launcher they spawn really slow for some reason so I suggest meleeing (however you spell that) them. You can get about 2 melees with those while waiting for respawn. Also, the Plasma rifle is touchy, aim for your partners feet or slightly in front of their feet (where the reticle is NOT red) or else the bullet seems to just pass right through them wasting double ammo and time.
If you have all weapons checked off and still no cheevo poped, do 1 more round and about 5 or so kills with each weapon. Might need a little more than 400 exactly since the cheevo says "more than 400 kills each). If you play campaign you probably won't even have to do all this, I skipped most of my pistol, machine gun, and rocket launcher since I played campaign 2 times all the way through when I did this cheevo.
My buddy did this while I was at work and knocked out about 3 hours worth of kills I wasn't even there.. Please Vote positive for this sollution since no one else has a guide like this :) BIG THANKS to ConsiderPHLEBAS for figuring this all out with me and boosting like a mutha so we could both get this LOL :)