Here is the full list of all 27 GUNBIRD achievements.
Clear the game Tetsu
Jungle stage clear
Clear the game in Valnus
Clear the game Ash
Clear the game in Marion
Clear the game in Yuan Nang
Final stage clear
Monument stage clear
Life increased
Ocean stage clear
Castle stage clear
Village stage clear
First stage clear
Mine stage clear
Upgraded shots to the maximum
Bad ending in the final stage
Ending without continuing in MONKEY mode
Ending without continuing int CHILD mode
Ending without continuing int VERYEASY mode
Ending without continuing int EASY mode
Store bombs to the maximum(Default setting)
Earn 50 Bonus Items
Clear the game without destroying any enemies
Achieve all achievements
Ending without continuing int NORMAL mode
Ending without continuing int HARD mode
Ending without continuing int VERYHARD mode