Again not a difficult solution:
note:If you have enough money to upgrade the sniper with the x2 zoom i could reccommend that.
Pick the sniper class and go play some TDM or Annihaliation
this are the best game modes to earn 100 kills quick.
Now there's 2 things u can do :
1.pick a high point and camp from there to stack up kills
2.pull your pistol out and go for some real dangerous pistol fights.
took me about 3 hours or smting to get it ... probably because the damn server was down all the time.
GL