Here is the full list of all 24 Crazy Brain achievements.
Finish an entire Insanity Rush Marathon
Smash 10000 Boulders
Crush 10000 Blood Clots
Smash 5000 Frozen Boulders
Splatter 5000 Freaks
Smash 1000 Eyes
Collect 1000 Silver Coins
Collect 1000 Extra Brains
Reach a Total Score Count of 4000000 points
Reach a Total Score Count of 5000000 Points
Reach a Total Score Count of 8000000 Points
Reach a Total Score Count of 10000000 Points
Finish a single stage with 300000+ points
Accumulate 500000+ points in the Tower of Madness
Accumulate 1000000+ points in the Tower of Madness
Accumulate 5000000+ points in the Tower of Madness
Accumulate 10000000+ points in the Tower of Madness
Reach a Force of 1000+
Reach an Intensity of 1000+
Achieve a clocked time under 5000 seconds for a full completed Insanity Rush Marathon Run
Spawn 101 Brains
Spawn 501 Brains
Spawn 1001 Brains
Spawn 2001 Brains