Here is the full list of all 7 Crystal Defense achievements.
No damage to crystal in level 1 on hard difficulty.
Win level 1 on easy or normal difficulty.
No damage to crystal in level 2 on hard difficulty.
No damage to crystal in level 3 on hard difficulty.
No damage to crystal in level 4 on hard difficulty.
No damage to crystal in level 5 on hard difficulty.
No damage to crystal in level 6 on hard difficulty.