I am facing this issue recently, after investigate the culprit was the Facebook app, i found the 4core cpu of Cortex-A57 still running at 1300mhz instead of 700mhz when the Facebook app is closed, the temperature of the CPU is heating and drain the battery extremely fast, what i did was go to developer options in setting, under Apps, enable 'Do not keep activities', now the heating & battery draining issue is gone.

Bro, we have the same issue. Can use my phone for 4 hours tops! 

Your battery dying. Mine also shutting down at 20-30%. Replaced new battery everything's good.

Victor3010

Mm.. I'm not sure if I fix it.

I followed someone's suggestion from reddit. They assume it's the battery calibration problems. I tend to start charging at 60% / 70% / 80% & even 90%. Not sure if this is the root of the problem.
Anyhow, they suggest to;
1. Charge your phone more than 1-2 hours to make sure it's fully charged.
2. Use it until 0%
3. Fully charge again.
4. Use your phone this time to see if the problem reoccurs.
I did the steps once and it's back to normal.

Today, I am gonna try again to see if the problem occurs when I use mobile data the whole time.