Step 1) Reset your stat points from the hobo in the city near the hospital he will be in an alley way and it will cost you 800$.
Step 2) Fight Kisuke and make your strength 200+ so that you can kill your bankai quicker.
Step 3) Unequip all your moves so the bankai wont be able to use them against you, he will still be able to use the shikai moves.
Step 4) Make sure you hug the wall so he doesnt run off and is predictable to attack against.
Step 5) Repeat until you master your bankai, i think you have to do this 15 time.