Yeah, people keep saying Flash is top 3 but once you play him you figure out he has hard execution and risky specials.
Try to play footsy using his backdash + b22f3 sonic pound. Your best whiff punish is the charge. In corner you can abuse d1d2, b22, d1d2 pressure that allows a crossup sonic pound corner combo (11 11 11 charge, f3 reset).
The far sonic pound goes very far and his charge allow you to do 30% from a d2 b22f3.
trait combo for beginners : starter, LK/SP MB trait, LKx3, b22 LK, b22f3 charge.
1 trait : starter LK/SP MB, 11, b22f3 charge.
Charge MB : d2 or walk forward 22, b22f3 charge.
don't forget : 2 dash and you rise the fullscreen.
Practice.