Rapps's attack pattern loops every 4 turns, it goes like this:
1: Wing Cut (possibly twice)
2: Wing Cut (possibly twice)
3: Scorpion's Tail (possibly twice)
4: Aero3 (if MP >= 50)
However, the 'turn counter' starts at a random point, so he can Aero3 you right away (I've certainly had that happen to me.)
So if you get lucky and have it start on '1', then kill him in 3 turns, no Aero3.