I am up to the part when atton gets in a fight with twi'leks and the fight is impossible to win, is there some way to completely avoid this battle?

No, you must fight them. Personally I didn't have too much trouble with the battle. I don't remember using stimulants, but I certainly remember using a shield generator. Tossing a few stims in there might make it easier.

 

If you still have trouble, another poster mentioned that he ran behind the counter and just shot them down from there. The path finding supposedly does not allow the sisters to go around.