Instructions
Switch weapons frequently to gain experience points for getting a certain number of kills with each gun beginning with 25 and increasing as you earn the achievements. Once you get the achievement, switch to another weapon to complete the challenge, gain more experience and improve your shooting skills.
Focus on using different attachments for each gun. Some attachments require you to get a specified number of kills with a lower-level attachment and give you experience points when you unlock them. Improve your weapon by reloading faster, getting a scope or having a grenade launcher attached.
Complete the challenges. Some challenges have nothing to do with your weapon but focus on other accomplishments such as blowing up vehicles or falling a certain height without dying.
Gather a group of friends and practice together in a private match. You won't get any experience points for playing a private game but you can focus on team-based strategies to improve your skill. Play together as a team in game modes such as a team death match or headquarters to win and get the maximum amount of experience. In headquarters, you get points for winning but also for smaller accomplishments including capturing the headquarters each round.
Play search and destroy when your skill level is up to par with some of the better game players. You get the most points for kills in S&D but only get one life each round, making it even more important that you act before your opponent to kill him before he has a chance to pick you off.