All You Need Is
You need to use the latest version of SuperSu and hopefully it will work out well for you.
I haven't done it before because am still stuck with android 5.1 but it should work well on your marshmallow device as it has been proven to work on others like it.
Temporarily Booting TWRP
Download SuperSU-v2.46 to your device.
First of all, Boot into TWRP temporarily, if you have flashed the TWRP skip this step.
Restart and then boot into TWRP
Tap on “Install” and select the SuperSU zip file that you downloaded to your device. “Swipe to Confirm Flash” at the bottom of screen to begin the flash. This will install SuperSU and allow root access automatically.