iPhone 6 turns off at 30% ; water damage

battery is power rail is probably shot. check the components on the battery power rails with a multimeter. clean off board and do ultrasonic cleaning. have bga chip xrays done, if your ocd.

I am working on hacking the touch id pairing, but until I succeed, your touch id is shot forever. only the mighty apple men (and women) can fix it

Feel for hot spots on the board while charging the phone. Like he said, there is a good chance the battery is damaged. Cleaning the board with a tooth brush and isopropyl is a cheap alternative to an ultrasonic bath. Remove all of the shields if you have a hot air rework station. Any hot spots would indicate a short in the area, look as closely as possible at the components in that area to find and replace any damage.