I have L1 as A and I did that as I don't want any unfortunate situations after it (plus I dont need shoulder buttons for A+B, or any other bind like that (B+K, Throws) I do them all manually by pressing two buttons always, at least that's how I got used to doing them).
So what I do is have already my thumb quite bent over A and G when it comes to iagA, and press A with the tip then quickly stretch my thumb, so that the upper side seems bent and during that, a part of the bone which comes out hits G. Then I quickly press L1 with my left hand's index.
When I was first learning iagA I couldn't do it at all, then I did it once, then twice and now in SCV (after 2 years of iagA-ing) I think I'm doing this with a lot of concistensy and TBH I had no problems doing iagA when I first played SCV.
I only find difficult doing this after a specific combo online though. I think I'm doing it too fast because of the tension and because the timing is quite strict, I mess it up more so nothing happens or I get A or a throw.
As for any kinds of pain, yes I do have. If I do iagA a lot my thumb hurts and if I play for too long my wrist husts a bit...
As for Slayer, some time ago he told me that he bought an arcade stick and doing iagA there was easier for him as all he had to do was to slide his finger through 3 buttons. I guess he must have registerd on his stick A G A, so that he would get to slide his finger on a staight line. He also said that he was feeling some short of pain while doing this and he'd probably need bandages or gloves, I dunno...