>>358729167
Best way to do skirts is to keep the original weighting. Most skirts will have a bone in the centre; split the weighting of that in half and add each half to the bones beside it. The goal being that you have bones above the legs which carry the weight of the skirt. 
Then in unity, constrain the skirt bones to the leg bones, so that when the leg moves up the skirt also moves up. 
IT's not as fluid and perfect as something more dynamic but it's performant as hell and looks decent. And more importantly isn't hard to set up. 
Someone here made a video on it... I'll try and find it.