I still REALLY question the practicality of trying to punish Deathstroke low guns with reversal FTD. On joystick, at least, with either alternate or default controls, it is REALLY hard to buffer the motion with the right timing without either getting hit (too early, dropped low block and got hit by 2nd shot) OR missing the punish (too late, DS recovered and could block for a free any-range punish with quick guns).
And I stand by my assertion that you really SHOULDN'T need that punish to beat even GOOD Deathstroke players (and I have played some SMART Deathstrokes). As long as you advance cautiously and stage smart mixup with full conversion (don't drop any combos you start) when you eventually get in (and you WILL get in, eventually, if you don't foolishly rush when you KNOW the DS player is ready for it), you should do more than enough damage on the inside to make up for whatever chip you had to take on the outside.
This is not a mission-critical punish to win the matchup, and honestly seems WAY too dangerous to attempt in a serious match IMO. If you mess up, you get HIT by guns, which leads to more guns for free (frame advantage), and the whole concept behind beating DS with Aquaman is not to get hit by guns, period. If you don't hit by guns, you will eventually creep in, and you will almost certainly do enough damage on the inside before or if you get knocked out again to defend your life lead by blocking and creeping back in. If you feed Deathstroke free gun chip because you insist on getting hit by low guns trying a flashy (and minor, unless you're at the absolute tip of dash b12 pickup range -- in which case you almost certainly have easier and bigger counters for whiffed or blocked low guns available) punish, you're playing his game, not yours, and you deserve to lose.
EDIT: I wouldn't attempt this punish offline, either. If you play as Deathstroke in training mode and watch the Aquaman dummy block and reversal punish, it LOOKS easy. But I assure you, when you try it yourself against Deathstroke dummy, you'll find it's MUCH harder than it looks, at any range. Do yourself a favor and just take the free dash on block (or b12, etc. if for some ungodly reason Deathstroke does it right in your face). If you want to SERIOUSLY limit Deathstroke's options, you need to get closer to him, not waste time at full screen trying pitifully to beat his gunshots.
And in case anybody gets the wrong idea, I think this matchup is in Aquaman's favor. Like, 6-4 good. Absolute bleakest-case scenario, it's 5-5. I think it was this good for him BEFORE the nerf to low guns. You have to respect DS's guns, but once you get in, you don't have to respect much else about him. And I think he's a lot easier to approach on the ground than characters like Superman or Green Lantern simply because he doesn't have air projectiles that go downward (and in Lantern's case, control a really annoying angle for Aquaman's jump + dash mobility). You do NOT need to be technical to beat Deathstroke. You just have to be smart and finish your BnBs.