Getting this achievement without boosting will be difficult. Getting together a match with enough people willing to play without respawns will be one challenge; killing 5 people without dying is another. It you have teammates - and I assume you do if you're playing against at least 5 opponents - the final challenge will be playing conservatively enough to survive while playing aggressively enough to beat your teammates to the kills. Remember, no respawns means if you're playing against 5 people you have to get every kill in the game. So if you're going for it legit, I wish you good luck.
For the rest of you, this is fairly easy to get on the road to boosting your thousand kills for the achievement "Don't Mess With Me". You need 6 unique gamertags for this achievement and several others in this game. Set up a player match with 10 slots, team deathmatch, 9 private slots, and pick any map. I like import/export, but for this particular achievement it's irrelevant as long as you can find the other team.
In the advanced settings, set respawns to none and random respawn to no (this puts everybody in the same location). Duration shouldn't matter unless you get really, really lost. Start up the lobby at this point, then go into match settings and change Lock Teams to no. Now invite the other gamertags into the room. Assuming you have a total of 6 gamertags, put five people on one team and the person getting the achievement on the other (this is only possible because you've allowed 10 people so the game caps people-per-team at 5). Start the match, track down the other team, lob a grenade (actually, C4 if you've got it - trust me, you want the extra assault points), and you should pop the achievement. The match will end immediately since you just killed the entire team. Move the teams around for the next person and repeat. You should be able to get 6 people the achievement in about 10 minutes.