What are the most painful punishment for a painslut, I have tried rubber snapping on her and wants to cross the bar it created in terms of pain
This is so vague I can't see how anyone can respond meaningfully.
You tried "rubber snapping", I assume you mean rubber band snaps? If so where, what type of bands, how much pull back, single snaps or repeats in the same place? Were they self applied or did you use them on her yourself? They are a very versatile item and can range from a mild sting to full on bruising welts depending on how they are applied. They can simulate a cane or thin strap effect quite well and it seems unlikely you've exhausted all possibilities with them.
Without knowing exactly how they've been used there is no way to suggest something that would "cross the bar" as you put it.