Here is the full list of all 9 Bullets Squared achievements.
Reach a Combo of 500
Collect 10 Gold Medals (any game mode)
Pass all levels, by earning at least Bronze on all levels' normal game mode
Reach Square Level 4
Reach Square Level 3
Reach Square Level 2
Reach a Combo of 250
Achieve a Platinum Medal
Reach a Combo of 100