I would buy a bottle of pool acid, cut it 50/50 with water, squirt in a good bit of dish soap and have at it. But I have all the tools and years of experience behind me.
You only have to rinse, no scrubbing or very little. I use a plastic water can with a sprinkler type head on it. As the acid runs down the wall it instantly removes all the calcium it touches. If it is stronger than 50/50 it will also take off any nasty chrome you might have. I keep a hose connected to the (open) bottom drain on the spa and another hose running the whole time to immediately flush the acid out of the bottom as soon as it works it's way down. Rinse well before you fill, but don't worry if the pH is real low when you refill - that will help a bit cleaning out the plumbing. If you don't have a bottom drain, use a pump. If you don't have all this stuff, just hire it done - might be cheaper in the long run.
[size=18]A set of old clothes, eye protection and some way to absolutely not breath the fumes are a must. Add acid to the water, never add water to the acid.[/size][/color]
If you can't afford/borrow a gas mask (this is not a joke in any way) then at the very least put a strong fan or two in place to blow the fumes away as you work. The fumes can kill you, and even if they just sit around you they can irritate your eyes and skin.
HTH
8-)