Note: Since you bought this game on Steam you already have Operation: Code Breakers DLC which essentially is just another minor mission.
Just start the mission on Rookie difficulty and complete it without firing a weapon or using a grenade. You can and you must use all of the special abilities of your teammates as well as yours. Moreover you can melee enemies as the last resort if they come too close. It's very easy. I did it with Recon and Commando at Level 3 both of them.
*** Minor Spoiler: What makes this mission very easy to complete (achievement wise) is that this is the only minor mission out there with no BIG enemies like Mutons or Sectopods!!!
Enjoy!