Here is the full list of all 15 99 Seconds achievements.
Complete arcade on easy mode
Complete arcade on normal mode
Complete arcade on hard mode
Get an extra time item
Get a bonus v99 item
Get 20 extra time items
Get 32 bonus v99 items
Don't take damage for 40 seconds
Get 12 Extra Time Items in a row (Normal or Hard Mode)
Don't use slowdown for 50 seconds (Normal or Hard Mode)
Get 9 bonus v99 items in a row (Hard Mode)
Get all 5 special items (Hard Mode)
Get 3 million points on Easy Mode
Get 5 million points on Normal Mode
Get 7 million points on Hard Mode