What I like to do if I think my horse will shy a bit when bathed, is work them beforehand. So I suggest giving her a good work out on a hot day, then she'll love you for bathing her because it'll cool her down.
We usually just use normal humans shampoo. So if she's got dandruff, try anti dandruff shampoo, that always worked great for us.
Also, many people seem to think that its best to spray the water as lightly as possible, only I've found it only makes the horse shy more. Maybe because it tickles them or something? I don't know, but that's what I've found. But I definitely think you shouldnt have much pressure on the hose. You need a little, but not too much because you want her to enjoy the bath right?
Hope it helped, have fun with Jubilee. If that's her in your picture, she looks gorgeous