Take her out of the barn where you'll have some room to run around and get your hose. She is rearing because you aren't letting her move around and you're making her terrified, you have taught her to be terrified as every time she gets anxious it leaves her alone. That is rewarding that behavior. Do this by yourself, it can be a tangled mess if you try and do it with two people and the timing will always be a bit off. She isn't being a brat. She has no idea how to behave and you need to teach her.
You've got to understand horses here, they are prey animals and their first instinct is to run away from scary things. So far you have taken that away from her and the only thing left she has to do it fight it. If you insist on putting her in a small space or tying her, you are asking to be hurt.
I use a spray nozzle and set it on shower, it's a light spray that's consistent and goes a ways so she's always be in spraying distance. Take your horse and just keep spraying her, starting on her top line not her legs. The topline is where everything else starts with desensitization and this is no different. I doubt she has a panic attack every time it rains.
When she freaks out and runs around just bump her nose back toward you so she can't get a lot of momentum built up. The important thing is you ARE letting her move around, you've already found out when you try and make her stand still she gets defensive of herself and rears up. During this you'll be standing at a 45% angle to her shoulder, too far back to be struck and too far in front to be kicked. You are working with her here, you are letting her move, giving her that option if she needs it but you're keeping spraying her. After a bit of a panic she will see that running is not working for her. A horse can only stay worried so long before they try something else. It may be backing up, it may be standing still. As soon as her feet stand still one second take the hose away. Horses don't learn from pressure, they learn from the release of it. This is why you take it away, you are telling her 'good girl'.
Start again, building that until you can spray her withers and back, then go to her butt, up her neck and then down her legs. After she stands still when you spray, wait until she relaxes then take it away. When that side is good go to the other. Horses brains do not transfer that sort of thing so start over, starting with her topline. In one session you should have her pretty well desensitized and able to be washed. This doesn't mean the next time you can tie her up. You want her to consistently be able to get hosed off without getting anxious. I would hose her everyday for a few days, then she will be ready mentally to be tied up while you do it.