CastleStorm

This achievement requires you to play through the majority of the Kingdom campaign and a portion of the Viking campaign to unlock all of the maps required.

After that it is very quick to unlock the achievement.
Fire up the map in Survival mode, kill an enemy to get some score for the leaderboard then just let the enemy break down your gate and steal your flag. They will carry it back to their base and level ends.

(If you have played through the campaign enough and have some more powerful weapons like the bombs unlocked you can give the enemy a hand by firing down and backwards at your own gate to cause some damage to it or even break it down yourself)

Took me about 10-15 minutes in total for all maps

To speed this up, you can create a custom castle with only one room, right by the gate. I used the Golem room (Kingdom), as it's a large target with thin walls.

The best thing about this is you can actually blow up the room *yourself*, by firing an explosive apple at your own gate. Total time is 20 seconds per level (plus menus).

Additional notes:

1. The achievement only counts "Survival" - you don't need to play "Hero Survival".

2. You need to play the first eight missions. The achievement unlocks as soon as you lose the eighth one.

3. You do actually need to set a positive time for each level. You can't just use a castle that falls down by itself.