Jack of All Trades - 25g
Play 10 Multiplayer games using each class
At first, the wording on this achievement can be quite confusing. It is not supposed to be read as "play 10 games using only each class" but it is in fact to play 10 games in which you play as all 3 classes.
So, just make sure that in 10 games you play as all three classes: Tactical, Devastator, and Assault. When you die, switch to the next class. For me, I'm the most comfortable with a Devastator and least comfortable with Assault. So, I began each match with Assault, then switched to Tactics, and then ended with the Devastator. This way, I could still contribute to my team, and perform the way I wanted to. At the end of the 10th game, you will be rewarded with your achievement.
Lastly, I found this to go much quicker in the Annihilation game mode than Seize Ground. On average, my Annihilation games were running 5 minutes, where Seize Ground was taking 8.
Good luck and happy hunting! If you're going to vote negative let me know why!