Here is the full list of all 24 SUPER FLAIL achievements.
Encounter The Boss
Kill 200 Enemies
Kill 100 Enemies In One Game
Die 10 Times
Unlock All Special Flails
Defeat The Boss
Reach 100 Combo
Reach 300 Combo
Reach 500 Combo
Shot By A Bullet 10 Times
Die 50 Times
Defeat The Boss 5 Times
Kill 5000 Enemies
Die To The Boss
Defeat The Boss On The Insane Mode
Defeat The Round 2 Boss
Kill 50 Enemies With Explosive Skull
Block 100 Bullets With Pulse Flail
Slow 20 Enemies At Once With Pulse Flail
Kill 20 Enemies With Electric Shock Simultaneously
What Are You Doing?
Hit One Ghost Skull 10 Times
Select A Random Special Power And Never Use It
Try To Use Special Power After Using It All