They haven't done battle. I thought they were all fake battles? Illusions, robots, etc.
They did battle as far as I can recall...
Skaar walked away from a battle once when Banner turned into "Child/dumb" Hulk... Skaar wanted to battle Worldbreaker Hulk... Not the mindless one.. He got his wish at some point and got his but handed to him...
Again... If I recall.