Operation Neptune
This achievement was changed today, it used to be 3,000 Medals required but is now 2400.
If you want advice on what units to use its simple, use a defensive set-up or offensive set-up and stick with it. Continually upgrade those units and fight, much, much fighting. I do not know exact details but there was an update that instead of losing the amount of medals your enemy gains you now lose less medals than the enemy gains from winning so its much easier than it used to be.
Units not to use:
-Gurkhas
Used for defense but not worth the 6 CP. Epic? more like eh poop.
-Flame Tank
You might think this is SICK but its really SHIT. Its basically good for defense but nothing else, a bigger version of the flamethrower but absolutely unnecessary since it costs 6 CP.
-Rifleman Squad (Epic version, not the white colored one)
This is possibly the worst unit in the entire game, definitely my least favorite. Reason? Everyone always has anti - infantry (like grenades) and those can kill a squad with half the CP. The only 2 times this unit is useful is when the enemy is distracted and used all their CP, or is split up by spawning directly in the middle of your 2 bunkers.
The main qualities about the units above is that
1) They are EPIC (when they suck).
2) They all cost 6 CP when they really aren't good. Use cheaper units if you need to defend yourself. you can drop them quickly and where you need them (Makes a world a difference).
So do yourself a favor by sticking with the same Strategy, not using the units above.