Astaroth is really scary. He punishes you like no one else.
Long range, you are in trouble. Close range..also lol but you can attack him.
The astaroth I fight is really aggressive. He keeps me far away with 3KA, 22B, 4A, bullrush² and when he gets close, it's iThrows, 6K, AA, GI. Hard to do something. On ground, I stand up immediately or else I take severe damages. I beat him 1 or 2/10 lol.
I don't advise stepping. It's anti-step are quite fast with good damages and RO options.
About iGDR, you really need to do it at the good time, else it's 70-80 dmg for you =).
Hum I don't know, try to varie your games, spam little attacks.. :|