Here is the way I learned how to do it.Hope it helps. Go to practice mode , do bloodnado and hold up. That way you'll know when the move recovers , and when you can press b3. You'll want to do f4 slightly before db1 recovers.
Tip #2
Make sure you press b and 3 at the exact same time. If you mess it up even a little atrocitus will step back before doing b3 and it will miss. Check your move notations to see if your input is correct.