Turok

For this achievement you need to kill five hundred enemy players in online "ranked" matches, these can be done in any gametype but "Free-For-All", "Team Deathmatch", and "Capture the Flag" seem to be better for acquiring more kills. The more you practice the better you will become and the kills will rack up faster so stick with it and find some weapons that you are good with. Once you kill the five hundredth player, the achievement will unlock.