Short of removing the battery, which is not supported with this device, the only other thing you can do is let the battery drain completely, so that the power button will not activate the screen. Then, plug it in with a high-quality Android charger (in my opinion, the one which came with the phone stinks, so don't use it). I used a charger cord from my old OnePlus One. This eventually did the trick. After about an hour, the phone was at 25%. I was then able to turn it on normally.
Hopefully, this drastic measure works for you as well....