There is 1 more secret achievements in sonic A.I.R. is doomsday zone in act select, first, complete the game with good ending or bad,then, you can come to the doomsday zone in act select without lvl select.
In fact, to make it even easier, all you have to do is enter the original level select and select Doomsday Zone, so the achievement will automatically be unlocked.
I Was Struggling And I Tired And It Didn't Work How Do I Do It I Know It's Up Up + Down Down + Up Up Up Up Really Four Times And Then I Tried To Do It And Still Didn't Work