CastleStorm

Can be unlocked in any level or in skirmish, even the initial training level in the Campaign.

The soonest you can unlock this will be in the first/tutorial level in the campaign. Go through all the basic stuff at the beginning until you get to the part where the game tells you to destroy the enemy castle. Aim slightly above the enemy castle's gate. Keep shooting your ballista until your Apple Bomb and Tri-Stone shots charge up. You are aiming for the rooms on the bottom 2 levels. Knock out the first 'column' with whatever you can. Once that section is out of the way, send your apple in and hit the next 'column' of rooms in those 2 levels. Then send in your stones, hit A right before they get underneath the castle, and watch rooms crumble!

Achievement unlocks as soon as you crush three rooms at once.