I am sorry to hear that you are having problems with brushes because hair is really difficult in vector form. On the plus side there are a huge number of of free hair brushes that you can download and quite a few tutorials so have a look here as I have used these: http://namespace.deviantart.com/art/...h-Kit-50284325
Tried to find tut for you on a similar style as Merida this one is the best but it is B/W: http://blogs.adobe.com/adobeillustra...ized-hair.html
The normal way is to start with the darkest area and just use the Pen Tool here and work out then use the hair brushes like in this tut but they don't have the wild hair that you want: http://crazychild.deviantart.com/art...-Hair-14251697
You have taken on on a difficult project but welcome to TG. There are a few here who will always give you help if you need it. If I was doing this project I would take it in stages take days off the drawing so not sicken your self.