Very simple!
1. Play missions to collect 5 medals
2. Go to Operation 4: Blizzard and unlock Zombie map
3. Play until wave 10 without any units dying, walk up to the 4th unit and the achievement will pop
As soon as it pops close the app down. Restart and you will have your 5 medals back
NOTEIf any unit dies don't worry they will spawn in the coming waves. In all honesty it's not that hard to survive, just move around the 1st and 2nd areas as these have the most space.