From the sounds of it, the body hair is matted? That's pretty unusual if that's the case. I'd use scissors to cut into the mat to split it and then see if I could curry it out. If you're talking mane and tail, then you can dip the tail in a bucket of warm water and shampoo and condition it real well, then use Cowboy Magic on it, rub it in and let it sit for an hour or 2 and then go out and start picking the mat apart with your fingers. For the mane, I'd use warm towels to wet it down and I'd just rub the Cowboy Magic in and again, let it soak in for a while and then pick the mats apart with your fingers.
Alternatively, if it's too cold for you to stand outside picking mats apart, cut them out and let her mane & tail grow back til spring time, you probably won't really notice.
If it really is body hair that's matted you can cut those or clip those out if she'll stand for it and just leave short patches in her body coat which you can cover with a blanket. Or, if they don't bother you too much, leave them alone til she sheds in spring, she'll shed them out.
If you want to do an all over bath, even in freezing weather you can (except if she's underweight, I wouldn't do it then). Just put a wool cooler over the horse and expose only 1/4 of the horse at a time. Wash and rinse and condition 1/4 of the horse, then rub as much moisture out with a towel as you can, then I take a blow dryer and just get the horse used to the noise for a few and then I stick the nozzle under the cooler (I've even used old wool army blankets if my coolers weren't handy) and make kind of a warm 'tent' and let the warm air circulate until that 1/4 is dry. Then you can move on to the next 1/4 and so on.
I used the army blanket & blow dryer trick on a horse who fell into the pond (he trusted the ice and fell through with his blanket on) and he warmed right up and felt very pampered when we were done. I followed up with a nice warm mash to spoil him even more. My husband came home in the middle of the dryer job and said he was shocked that the horse wasn't standing in front of the fireplace in the living room with me drying him off. I didn't tell him that I thought about it but figured that might be THE final straw as far as him and the spoiling .,...er care of the horses went!