This achievement requires you to be Level 25 or higher. Just continue playing games and the achievement will pop. Some tips for you:
Play with a group. Playing with friends is fun, and grants a 20% xp bonus.
Don't leave. Leaving 3 out of 20 matches will leave you with a leaver's penalty which deducts 75% xp match.
Play a wide range of classes, find your strengths and weaknesses. Healers are easy to get a gold medal but don't do damage. Defenders are good snipers and deal high damage.