The Club

Although you will find similarities in the other solutions,(credit to them for those findings), this is an unattended solution for those of you with 2 or even 3 consoles.

Pre-Requisites:
Setup:
Sign in with your main account on your turbo controller and host a match with these settings:
Have the other console\s search 'Ranked' and join the host. Place all other players are on the Blue Team. Either choose 'Renwick' or 'Dragov as the Blue Team characters, DO NOT use 'Kuro' ^{(Credit to Mister Buds)}
In order to start the match, you need at least 2 players and balanced teams.

Once the match has started and the teams are setup properly. Move the Red player through the map to the Blue Team’s spawn. On the way there, be sure to shoot the (2) Blue skullshots on the side on the train car and the other (2) Blue skullshots prior to exiting the train station. Leave the last skullshot intact, so the match doesn’t end.

Kills:

After about 4-8 hours have passed, end the round to save your progress by shooting the last skullshot. It’s located around the corner, above the car to your left. Some people have experienced the game crashing over long periods of time, so be sure save your progress by ending the match from time to time.


While using this method, you’ll be able to get the following achievements as well.






That’s pretty much it. Let me be very clear. This is not a desired boosting method for three people, it is best for people who have the capacity to do it solo. You can do this with 1 other person, if you really wanted to take turns getting kills each night. That’s up to you. I completed this game entirely solo, and it was honestly the best solution for me. ...so, go and get some kills while you do something else!

There is another - even faster - way to get your 10k kills. Select Team Skullshot, 1 Round and WARZONE (not warehouse as all the other solutions suggest).

In order to start the game, you have to be 2,4, 6 or 8 people. Players from the red team will kill the blue team. This is quite a large level, so team red has to go to team blue and kill them.

There are 4 respawn points (as usual) - but they are all "at the same place". There is no need to move around - simply kill them as they respawn. I suggest that you first wait 1-2 seconds, then giv'em a headshot etc. There is always enough ammo - simply walk over the dead bodies.

Note: In order to have always 2 players to kill, start the game with 4 players. As soon as the game started, one of the red team can leave. The other one can go for the kills.

With this method, you can have 500+ kills within an hour (2 blue players) or about 260+ with only1 player in team blue.

[EDIT] See the comments of planting42 and AC1DGro0ve 3o3 for playing with 3 people. Thanks to both of you :)

[EDIT] The achievements pops right away when you reach 10k kills (thanks finallife6 for confirming)

Thanks to SAZorlance for the general strategy, but in further detail I'd like to add the following:

IMPORTANT NOTE: in february (2010) SEGA closed down the PS3-servers for this game so it's quite likely they'll shut down the servers for the 360-version as well. Time to get started before it's too late ;)

IMPORTANT UPDATE: the last 3.000 kills I did on the map Warzone. Just run to the other end of the map to find 1 fixed spawnlocation (actually there are 4 REALLY close together). It doesn't make things faster compared to the method described below (still approximately 250 kills per hour) but it's more relaxing since you don't have to run back and forth for every kill. You can just stand there and move your crosshair around. Hope this helps.

ORIGINAL STRATEGY I STARTED OFF WITH:
The headshot trick while playing the Warehouse level on Team Skullshot definitely doesn't work, but it doesn't really matter at all. Then again, it's a great way of boosting this way. If you host the match you can boost this semi-passively since your fellow-boosting partner doesn't have to be present and/or actively play. He just needs to leave his xbox running when it's his turn to get killed. This means that the amount of time it takes to get to 10k kills is split in half (when switching back and forth you'll only need to actually play when it's your turn to kill). I was quite relieved when I found this out!!! :)

Furthermore, when you host the match the other guy is spawned in 2 different locations mentioned earlier (in the garage and alley). On both spawnlocations there are actually two spots he can spawn on (they are really close together like a couple of steps away). So when you kill him in the garage make sure you can see both spawn locations in the garage (pretty straightforward if you position yourself just outside the garage). In the alley you have to position yourself a bit more accurate since the two spawn locations in here are further apart from eachother so you need to cover both angles. So why is this important? Well here goes: as soon as you kill your boosting buddy you'll have to wait 10 seconds for his body to disappear. When his body disappears that's your marker that he actually spawned again on a different location. So while standing in front of the garage after you killed him and the body vanishes and he isn't spawned on the other spot in the garage you now know you need to run to the alley to kill him over there and vice versa. You'll notice that quite often he spawns a second or third time on the same spawn location (garage or alley) which means you can shoot him again straight away.

In addition: the killer needs to pick a strong and fast character (you need to run back and forth between the alley and garage and you need to kill fast) and the cannon fodder needs to pick Kuro (weakest guy and therefore quickly killed). You'll also want to make sure you make headshots for the fastest kills. Now you just need to take turns every 200 or 300 kills or something. You can easily get 250+ kills per hour this way (I've actually timed it and doublechecked it) meaning it'll take 40 hours to get to 10k kills. Not too bad since you won't have to help out your boosting buddy sort of speak (meaning you don't have to actively play it for a total of 80 hours). Finally, make sure you track your progress and write down the number of kills to keep you motivated since it can be a total drag ;)

A reference to the Gears of War achievement, 'Seriously', there's no easy way to get this. The quickest way I found was to play the Warehouse level on Team Skullshot mode where there are only two spawn points (in an alley and then in a small garage) where you can run back and forth killing the other player. As long as you kill him with a headshot he will always spawn in the opposite point, if you kill him without a headshot he may spawn in the same spot again, wasting a bit of time.

I was fortunate enough to have two 360s and could boost it myself, but with two people you should expect to put at least a month or two of playing to get this, but rest assure when you get your 10,001st kill the achievement will pop, no glitches at all. Good luck!

Actually if you have only 3 boxes available you can start the match up. Yuo just need to get the last account that joined the match you "sit in the middle" / undecided team. I never figured out how to get it thre I just jiggled the D-pad untill he ended there. Now you can start with 3 accounts and 2 people on blue.
And yes Warzone is so much better. I did Warehouse for the first half, untill I discovered the Warzone spawn.