Mafia II

I found the best way to grind kills was on "The Don" mission located at the docks. I switched back and forth between machine guns depending on which type of ammo the enemies were dropping (changes by the area).

Make your way through the enemies leading up to Big Sal. It may seem like they spawn infinitely but they don't. If you keep moving forward, many enemies seem to run to a predetermined spot before they try to attack you, so you can just pick them off as they run past. I stuck to machine guns and tapped Left Trigger rapidly on distant targets to snap target them (on Easy difficulty). I could manage the run up to Big Sal in less than 5 minutes, killing anywhere from 65 to 75 guys depending on where I ran.

Once you've killed him, start running back and once you get to the 3 tankers enemies will start spawning behind you. Take a few out then let them kill you to avoid the rest of the run back to the safe zone.

This method should without a doubt get you the achievement in under 45 minutes if you've completed the story, which I finished with about 400 kills.

You'll get this by replaying missions in the game. By the time I was done with all missions i had 600 kills. The remaining 400 i got on the mission Sound and Furry which has quick easy kills (Top right on map by Bruski).

Start it up and run forward towards objective, equip machine gun, kill 2 enemies on the right behind fence, cover behind white car, shoot the 3 enemies running in the far back, then the guy on the left, last one around the corner to the left. This will get you x7 combo. reload last checkpoint and redo. This will take between half an hour to an hour to get 400 kills.

!!! PLEASE READ THIS BEFORE GOING FOR MASSACRE !!!

As someone mentioned before, if you got Millionaire achievement with drop-through-the-floor-in-the-shop glitch, prior getting Massacre then the latter might not unlock for you (and Explorer as well). So this is possibly partial solution for Massacre not unlocking at 1000 kills.

So here it goes: I said it's partial because if you have save file with more than 1000 kills this won't work for you. Achievement system just tracks hitting 1000 threshold, not 1010 or 1200. (Bad coding example...).

So here's my "story" and what I did (step by step) to unlock the achievement.

1. I've got Millionaire achievement with above mentioned glitch.
2. I did not have the Explorer achievement at that moment.
3. IMPORTANT! I was replaying The Don mission to get 1000 kills, but I SAVED THE GAME (subs on/off method) when I got 993 kills. Let the enemies kill you at this point.
4. IMPORTANT! Started The Don mission again, but when I hit 1007 kills with NO Massacre achievement unlocked, I quit to dashboard without saving (just power off console or quit to dashboard from the game without returning back to menu or saving the game).
5. Just to be safe I copied Mafia II save file to Flash drive.
6. Started the game.
7. Selected Jimmy's Vendetta and pointed to Flash drive when asked. After game has started I checked that I still have 993 kills;
8. Pressed Continue and started The Don mission (can be any mission I think);
9. After exactly 7 kills Bang!!! achievement poped, saving me a lot of time and nerves instead of redoing it on the fresh save!

REMINDER: If you just started your way towards 1000 kills DO NOT FORGET to save somewhere between 990 and 999 kills.

Hope this will help to those of you who just going after Massacre cheevo or for those who have save file with less than 1000 kills.

This is confirmed as working method.

I do not have this achievement yet but I am very close.

But i found the strategy that worked best in the mission "Tam in a Jam" this mission is located near the top right of the map theres a little path leading to it it's the only mission around.

I found it easy because there are at about 40 of his guys and you will already have about 600 of just playing through to this level. I may take a while but keep replaying the mission until you get it and if you get bored there are some other missions where yoy can rack up easy kills.

Don't forget to take plenty of ammo and also when you die the people you killed before you died still count towards the 10000.

Vote positive if this helped and please leave a comment if you find another easy solution and i will update it.

Thanks :]

Here's something no one has mentioned, that I would've liked to have known going into the post-game kill grind:

Collateral-explosion kills via exploded vehicles DO NOT COUNT!

Like many, I was using "The Don" mission to rack up my kills. This mission begins with a group of enemies (and the initial waves) positioned near three Hank B trucks. I thought I was being efficient in starting the mission by luring them closer to the trucks, then shooting the trucks' gas tanks. It was a fun way to start things off (with a bang)... until I realized those kills weren't being counted. This means every time I replayed the mission before this realization (which was a lot, I'm embarrassed to say), I was cheating myself out of the first eight-or-so kills.

Don't make the same mistake I made; just gun them all down. Even though collateral-explosive kills DO give you an Explosive Kill tally that keeps the combo alive, these kills are NOT considered legitimate by the game's Enemies Killed counter... which is pretty lame if you ask me.

This Solution guides you not only to work on this particular Xbox Achievement, but on two more during the effort. I suggest you shall go for this once you have achieved 100% completion in Jimmy’s Vendetta.

Please view my created relative video first: http://YouTu.be/ccADmT5xBpg

Getting armed: Purchase Grenade Mk.IIs from either Harry's or Giuseppe's Gun Shop. While Harry's place is around the top left of the map, Giuseppe's is a bit to the right of the center on the map.

Now you can get ready to learn about the destination: On the top center of the map, you will recognize a B-icon in a blue circle - it is for Bruski's Scrapyard. Only a bit below from there, is the Mission, Sound and Furry, if unlocked by you. It is the Mission you shall play when going for these Xbox Achievements for Mafia II in Jimmy’s Vendetta:
-Faster than Light (very simple to do that way/comes naturally)
-Firebug (very simple to do that way/comes naturally)
-Massacre (very efficient method)
-Explorer (you might just achieve 1% of the task, when you are always using a car to drive these few meters only every run - but there is no reason for not doing it - besides, perhaps you want more cars around a zone where you destroy everything, like me!)

I would say, the process is going down pretty fast - I doubt that there is a greater method than this. If you think so too, please vote positively on both: Solution; Video.