Here is the full list of all 30 CountDown achievements.
Cleared 120 seconds of countdown game-mode in Co-op
Cleared 200 seconds of countdown game-mode in Co-op
Cleared 30 seconds of countdown game-mode in Co-op
Cleared 250 seconds of countdown game-mode in Co-op
Cleared 60 seconds of countdown game-mode in Co-op
Cleared 300 seconds of countdown game-mode in Co-op
Cleared 400 seconds of countdown game-mode in Co-op
Cleared 500 seconds of countdown game-mode in Co-op
Cleared 800 seconds of countdown game-mode in Co-op
Cleared 1000 seconds of countdown game-mode in Co-op
Cleared 500 seconds of countdown game-mode in singleplayer
Cleared 400 seconds of countdown game-mode in singleplayer
Unlocked Tonic
Survive for 500 seconds on Endless
Unclocked Vestal Cruiser
unlocked Allegiance
Survive for 250 seconds on Endless
Survive for 100 seconds on Endless
Cleared 300 seconds of countdown game-mode in singleplayer
Cleared 800 seconds of countdown game-mode in singleplayer
Cleared 1000 seconds of countdown game-mode in singleplayer
Unlocked Nisus class :Racer
Cleared 250 seconds of countdown game-mode in singleplayer
Unlocked Bolt
Cleared 200 seconds of countdown game-mode in singleplayer
Cleared 120 seconds of countdown game-mode in singleplayer
Killed 50 enemies with shield and unlocked a new ship
Unlocked Speedster
Cleared 60 seconds of countdown game-mode in singleplayer
Cleared 30 seconds of countdown game-mode in singleplayer