By dexstar setup do you mean calling him safely. Because if so D2 is your best bet, followed by 22. F3 splat also works well.
His pressure is basically based off of doing a string into vomit. F2 has the largest range but is slow, B2 has less range but is faster, and f1 has the least range (but still good range) and is fastest. Learning when to use them depends largely on learning frame data and figuring out the best options in each situation. In some cases you can do without the vomit like with f22 or go for a puddle mixup, but vomit is standard Atro pressure.
After vomit you have a few options. You can try and beat out their d1 with your own d1 for a full combo. You can try and use f1 to get more pressure going. You can jump to try and get a crossup. Or, and this is what I do a lot, you can just walk back or backdash and let them hang themselves with whiffed buttons. MB B3 is also an option.
The combo thread should be pretty updated with the optimal combos. His wakeups are bad so you need to really learn to mix your wakeups up (delayed get up, backdash, regular get up, charge, up vomit, regular vomit, etc.) Also learn his resets as they are a huge part of his game. Midscreen if you end any trait combo with b23 but shoot a blood ball right before doing b23, the blood ball will combo on a very late frame and leave you +30 (you get a free F2, they can't MB B3 or backdash it). In the corner ending in b23 vomit leaves you +15 or so unless it's on female hitboxes, in which case you're like +5 or 6.
