Cistern!acid is in fact its own special brand of horrible, and will eat right through magic bodies as well as flesh-and-blood ones! We'll leave up to you exactly what that feels like ICly. Ryouma may dissolve slightly more slowly than a flesh-and-blood person, but once the acid hits his skin, it will be near-impossible to wash off, and the whole process will be almost unbearably painful. It's not a guaranteed death sentence, but he may need to simply remove the affected limb to stop the acid's effects eating the rest of him if he doesn't want to dissolve completely.