The Slaj Jazz
I struggled against a good flash player a week ago.
Essentially they are just trying to get in and get you to block their b2 which leaves them at +4 on block.
So they can d1 into 50% combo or F2 into 50 combo.
A big part of deathstroke's game is the armor F3, like a player finishes a string that puts them at block disadvantage I armor F3 and hit confirm into full combo punish or on block I have a 50/50 overhead/low with b1 or b2.
The armor F3 strategy does not work against flash because if he just continually mashes b2, f2 then armor f3 will go over and whiff flash during those startup animations.
At the end of a flash string if they just hopelessly mash I would suggest a neutral jump 3 into 123 ninja vortex.
If they do not hopelessly mash I would use the armor F3 strategy but use it sparingly.
The same issue pops up when you try to just play footsies with F3.
So when I am playing footsies with any other character when they are in range I am F3 with MB vs flash that's a no no.
I started beating the flash player when I stopped using F3 as a zoning tool and started using JF3 I could not fully test the strategy because after I started using it, (and winning) the flash player quit playing me lol.....
Against flash I would J3 because his anti air is crap. J3 like an idiot. He doesn't have an answer for it.
I would always suggest using crossup jump ins on flash wakeup because if he blocks you have a 50/50 if he wake up attacks he moves himself full screen away.
Here is the difficult part of J3 if you hit flash with the very tip of J3 on block all your strings will whiff except 3 and B2.
So you can J3 B23F3 or just 3. 3 on block puts you at +4 on block but out of range to frame trap 32 puts you at -4. So those options at the tip of J3 are total garbage.
I don't think the D2 of Flash is good enough that it would require you to tip the J3 like Aquaman, KF, GL.
Also flash's MB charge is +3 on block if you duck the second hit of the charge it will whiff and is punishable in the air easiest punish is d2.
So just go for the big time J3 and frame trap with B2, B1, and F23. That would be advice based on the matchups I played.