Here is the full list of all 14 Bullet Witch achievements.
Cleared all stages on NORMAL difficulty.
Cleared all stages on any difficulty.
Cleared Stage 6.
Cleared Stage 5.
Cleared Stage 4.
Cleared Stage 3.
Cleared Stage 2.
Cleared Stage 1.
Cleared all stages on EASY difficulty.
Cleared all stages on HARD difficulty.
Cleared all stages on CHAOS difficulty.
Cleared all stages on HELL difficulty.
Played for over 15 hours.
Maxed out all power-ups.