Panzer General Allied Assault

I did this on the second level as that is quite easy, just get a unit that can move two spaces and as many artilerys as you can. Go and attack 3 other units while trying to make sure your artilerys are in range, since the enemy units are quite weak it wont take long.
The achivement will pop up along with unlocking a new card to use as soon as you defeat your 3rd enemy unit.
This achivement is alot easyer than it sounds and like Spiffing WotWot said if you cant do this try and find a frend to boost with.

Get a strong unit into enemy territory, and dig in, preferably on terrain that gives a defense bonus (forest, city etc). You'll get first strike when attacked by the enemy as you're dug in, so you should be able to defeat several enemies. Use combat cards and sacrifices to keep blasting through the enemy. Remember that for each enemy you defeat, your until gets +1 health.

Burn through combat cards to ensure that the unit survives, and sacrifice cards to boost defense if needed. Spend your Prestige on extra cards each turn so that you have things to sacrifice.

If you're having trouble, an alternative way is to boost with a friend in a Player Match.