I've been using a pumice stone, and a small river rock to tap at the heavy scaling (believe it or not, I have scale that is 1/16th - 1/8th inch deep near where the waterline was). The rock has put a few small nicks in the finish, and I do believe the pumice stone is scratching the tile a bit (renovating the pool is planned for the near future, so I'm not as concerned with a little bit of tile scratching or a ding or two).
I've seen suggestions to use a mixture of muriatic acid and water in a spray bottle - mind the blowback! For what to use to scrub, people have suggested sponges - and yes, it will eat the sponge. I think there were a couple of other suggestions, but I don't recall what they were.
A local pool builder suggested to me that I use a razorblade to get up behind the scale (when he saw how thick the scale in the remaining sections of the pool was, he said "yeah, no way acid is going to put a dent in that!"). My scale is a bit flaky, and he said a razor blade will do a great job of getting the massive amount of overburden off, leaving thinner deposits that the pumice stone will take right off. He did caution me to ensure if any of the blades cracked to get any pieces that fall into the pool out immediately - you don't want rust stains or a blade piece in someone's foot. I don't know what effect the blade will have on the tile finish.