Taken from StackExchange
Thanks to LangLangC
For temperature and other improvements see https://gist.github.com/cdleon/d16e7743e6f056fedbebc329333d79df
Shutdown, unplug everything except power and hold
leftShift + Ctrl + Option + Power
I holded for 5 seconds, but holding just for an instant should work
Command + Option + p + r
Until you hear the startup chime two times.
if you are on high sierra 10.13.6+ you might need to use
Command + r
instead
Command + r + s
csrutil disable
nvram fa4ce28d-b62f-4c99-9cc3-6815686e30f9:gpu-power-prefs=%01%00%00%00
nvram boot-args="-v"
reboot
Command + s
It might look as if it hanged, but press enter and you should see the shell (root#)
/sbin/mount -uw /
mkdir -p /System/Library/Extensions-off
mv /System/Library/Extensions/AMDRadeonX3000.kext /System/Library/Extensions-off/
touch /System/Library/Extensions/
reboot
It will show a bunch of text in the screen (don't be alarmed, let it finish) and then it will restart again In second restart it will show text in the screen again and then it will show normal login screen Your computer now should work properly (dGPU off and iGPU on, shown as i in gfxCardStatus
If an update that contains changes to the AMD drivers is about to take place it is advisable to move back the AMDRadeonX3000.kext to its default location before the update process. Otherwise the updater writes at least another kext of a different version to its default location or at worst you end up with an undefined state of partially non-matching drivers.
After any system update the folder /System/Library/Extensions has to be checked for the offending kext. Its presence there will lead to e.g. a boot hang on Yosemite and Sierra, an overheating boot-loop in High Sierra.
So I went back to square one and started again , reset the pram etc. I couldn't enter single user recovery mode so had to enter single user mode. I entered the various codes and everything worked great. Entered recovery and disabled csrutil in terminal and followed all the rest of instructions. I know it worked because all the nasty green lines are gone etc. However, now I am hanging on apple screen and progress bar is going nowhere. I know it must be a software issue but have no clue how to fix this.. Help appreciated.