Far Cry 2

the best way to rank up is to play team death match and revive often, this will give you extra XP even with out you killing any one. Just rember to make sure there are no guys around to shoot you while you rez someone.

here are some extra tip's:

.make sure to learn where all the most used sniper spot's are
.always try to keep a good amount of ammo
.if you are going to revive your team mate make sure there are no enimies around.
it's best to stick to one or two classes so you can perfect them, using to many classes may throw you off and cause your team to lose
.try to make sure you win at all time's it may sound obvious but you get extra EXP for win's.

also you do NOT want to reset your classes/diamonds. i did that when i was like level 7 (i accidentally pressed it whilst mashing all the buttons at the end of a game) and lost 3000xp. BIG MISTAKE. DONT MASH ALL THE BUTTONS AT THE END OF A GAME

To add to SkillfulMmd's solution, you need 15,000 xp to reach level 10. You get 15xp for every kill and revive. Don't teamkill or make suicides because you don't want to lose xp.