Here is the full list of all 12 Doc Clock: The Toasted Sandwich of Time achievements.
Fall into a pit of lava 3 times
Propel your vehicle 150 meters into the air, using any means possible.
Collect 20 robot heads.
Perform a 540 degree flip in a vehicle.
Destroy a robot using a robot head.
Destroy five robots in two seconds.
Sound the bell ten times within ten seconds.
Sit in front of a TV for five minutes.
The gnome must be cast back into the fiery chasm from whence it came.
Collect 75 robot heads.
Pick up twelve toasted sandwiches.
Complete every level without using the time slider.