This Achievement isn't Hard at all.
You Start with 10 Rounds on the Counter, but you can easily Complete this Mission in 5-6 turns.
For this Mission I had 2 Heavys , 2 Snipers , 2 Assault... but i guess it doesn't really matter at all, but you should have at least the 1 or 2 Snipers.
Simply Place your 2 Snipers on Top of the Train (Squad Sight Recommended) and Shoot everything that Moves.Rush trough the train with your Heavys and your Assault. Your Assault should take Point and Rush to the Driving Room while your Heavys place the Transponders. You have to Complete this Mission with at least 4 Turns on the Counter left! Also YOU DO NOT HAVE TO KILL ALL THE ALIENS! Simply place the Transponders and get one of your Soldiers inside the Driving Room. This should End the Mission.
On Easy Difficulty: 4-5 Thin Man , 3 Plasma Rifle Mutons