Here is the full list of all 31 Fixeight achievements.
Completed Stage 6
Completed Stage 5
Completed Stage 5 without assistance
Completed Stage 4
Completed Stage 4 without assistance
Completed Stage 3
Completed Stage 3 without assistance
Completed Stage 2
Completed Stage 2 without assistance
Completed Stage 1
Completed Stage 1 without assistance
First death of many achieved
Play a hundred games
Accumulate a thousand deaths
Pick up 100 bombs
Completed one cycle of the game
Completed Stage 6 without assistance
Completed one cycle of the game without assistance
Completed the game using a single credit
Completed the game with two players using a single credit each
Completed the game with three players using a single credit each
Completed a cycle of the game with two players
Completed a cycle of the game with three players
Completed a cycle of the game with Howard
Completed a cycle of the game with GX-028
Completed a cycle of the game with Vistario
Completed a cycle of the game with Agatha Bordeaux
Completed a cycle of the game with Lucy Pamela
Completed a cycle of the game with Cull Horn
Completed a cycle of the game with Hayate Zigragi
Completed a cycle of the game with Remlias L'Angelo