If you follow this solution, you'll unlock 7 achievements in about 10-15 minutes (for each player going for achievements), with just 2 players (you and a boosting partner) in players match, while if you don't use this guide it will take about 25 mins (each) just for the "Flag Frag" achievement.
From the main menu, select
MULTIPLAYER> XBOX LIVE> PLAYER MATCH> CREATE MATCH> CAPTURE THE FLAG
then, use these settings
MAP=Garage
FLAG CAPTURES=1
TIME LIMIT=60 MIN.
AUTO TEAM=NO
MAX PLAYERS=2
PRIVATE SLOT=1
Create the match and invite your friend/boosting partner to the match.
Select the skin you prefer, and as your weapon, choose
Sniper> M40A3
then press X to customize it, and on the second accessry slot select "Rifle Suppressor".
Now, the player going for the achievements has to run near his enemy base (where the enemy flag is, but not in the same room with the flag) and wait there (without touching the flag), while the other player has to go to the enemy flag, grab it, and return to his base WITHOUT deliver the flag, standing in the middle of his base room.
In this way the "losing" player after every death will always respawn near the flag (or in the same room!), and the enemy flag will still be there, so that he doesn't need to run to the enemy base after every kill.
The player going for the achievements has to crouch (pressig DOWN on the D-pad) and kill the rival by an headshot. Be sure to not miss any single shot, in this way after 5 kills you will unlock
and after other 5 kills you will unlock
From now on, it's not necessary to stay crouched.
Do 20 kills more, and you will unlock
Now you just have to collect the enemy flag and deliver it once, so that the match will end. This will also unlock
After this, you'll have about 1/3 done for
As consolation prize, the losing player will unlock
Not bad, uh?