Easiest done if you have at least 3 friends. Play Arena CTF Endurance and take the doubler. You can average 50 kills a game with the powerup. Or, if your friends are nice, they can host a party of three and search Arena CTF Endurance and you can by yourself, making it a 3v1. If they are good friends, they will let you rack up kills for 30 minutes as well as some easy flag captures. You can get around 150-200 kills this way a game until it gets boring. If you don't have any friends willing to help you out, Deathmatch Average Or Endurance will net you 15 or 30 kills, respectively, if you win.