First of all this MUST be done in ranked matches!
Playing Legit ~ If there aren't very many people online I would definitely host a match. I personally prefer Team Deathmatch but Deathmatch works as well. If you are going to play TDM then play on a map called Canyon, great map for kills. If you are playing DM then play on Camp! Learn the maps too better understand where people like to camp. I personally prefer the Light Recon class sniper, it has a 10 round clip and is silenced and can kill someone in 1 shot to the head.
(Playing Legit Time to earn 2,000 kills ~ Approx. 20-25 hours)
Boosting ~ If you are going to be boosting kills you will need atleast 4 people to start a match. I would suggest boosting TDM on Canyon and take turns getting 24-25 kills a match. Just cross the bridge and begin killing your friends as they spawn!
(Boosting Time to earn 2,000 kills ~ Approx. 15 hours)
Hope this helps!