First of all:
you need to find a trusted friend that will not mess around and that wont get kills then back out.

Second:
both of you will need to equip the same class which would be:


primary weapon:
basically any gun that is silenced and can protect you incase of any people find you ( this problem is very unlikely).

secondary weapon:
any silenced gun that can protect you incase your primary runs out of ammo.


equipment:
tactical insertion you will need this so you can respawn back in the same place and not the other side of the map.


special grenade:
any really because you don't need them.


perk one:
scavenger pro incase you run out of ammo you cant get more easily without dying.


perk two:
cold blooded so that the others in the game cant find you.


perk three:
ninja so others cant hear your footsteps.


death streak:
any just not painkiller or martydom


best killstreak rewards are:

i usually put on :
5 = predator missile
11 = chopper gunner or ac130 both really good
25 = tactical nuke

third:
Once you both have the class above go into a private chat with your friend. One of you will search for a game of Free for All. Once they (or you) get into a game the other person has to join session in progress when able to.

fourth:
Once both of you are in the same game you will need to find some amazing hiding spots.
you could find some by going on youtube and typing in: " ma2,nuke boosting hiding spots on (type the map name)

fifth:

Once you are both at the same spot both of you put down your tactical insertion then one person will kill the other. Once you kill him wait for him to put down the tactical insertion again and keep killing him. The best way to do it is have one person get all 25 kills one game then have the other person get the 25 kills the next game.

(i would really appreciate it if you could give me some rep but it is not reccomended)

hope that this post was helpful
max