Defense is the Best Offense is an achievement that is unlocked upon reaching Wave 10 on Pure Strategy mode, on all core levels (The Deeper Well to Glitterhelm Caverns), on Medium difficulty or higher.Earning this achievement unlocks the Pandora's Box Crystal Skin.
This will only unlock after a level has either been lost (by letting the Crystal be destroyed) or by full completion of the level.
Many players have reported issues with unlocking this achievement. The easiest way to acquire this achievement is to begin at Wave 8 (the highest possible starting level) and play until you reach Wave 10. Once you have reached Wave10, sell all of your defenses and let the minions attack and destroy the crystal. Remember, you do not have to complete Wave 10, you only have to reach it.
This achievement will be unlocked immediately upon the final levels completion, (wether it be full level completion, or the cores destruction at or after Wave 10.)