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