What I usually do is throw out a move with lots of active frames on it, pots bd has lots of invuln frames but has like a frame or two(?) of vulnerability at the end of it. So if you have a move that is active as his invulnerability is wearing off he can't do anything about it. I like to use 2d because it moves you forward and has more active frames that any other normal of slayers, you can also link a 2p after your 2d if it is meaty enough for 2p-5p-5p-whatever air combo. 2s also works with 9 active frames, 6h and mappa works too due to lots of forward movement, actually you can use anything to punish but you have to be precise about it, 2d is kinda braindead and leads to decent damage.
This situation usually occurs when I am using 5p as oki and he gets tired of blocking it so he backdashes inbetween them or as he is waking up. Its your job to realize your 5p whiffed and then just input 2d.
In other situations like when he backdashes a blatant jumpin or whatever, use your judgement. 5p and 5k are super good at stuffing at certain ranges.
In reality the safest and best (they go hand in hand in ggac) option is when you see his backdash, input your own backdash expecting a slidehead or hammerfall (youre going to be out of range for pot buster) and cancel your backdash with a 1 frame jump to avoid either option. fall straight down with j.h if hammerfall, airdash j.h if slidehead. K dandy works in place of the 1fj if youre feeling saucy and want to please your audience