If you installed Android 6 from locked Lollipop Bootloader or forgot to unlock the bootloader before the installation and all you need is to root the device, this is a guide for you.
I did unlocked my Bootloader when I was on 168 with Asus unlock app. I flashed 194 raw file via Asus Flash tool which locked my bootloader. I installed MM via adb sideload and I had MM with Locked bootloader. I used this method to root my phone.
I am sure you'd already known that rooting the phone (acquiring the administrative rights for your own phone) will void the phone warranty.
With this method,
1. You do not need to downgrade your MM bootloader to Lollipop, unlock it, install twrp and root the phone, and then upgrade back to MM.
2. You do not need to install the TWRP. You will be running the TWRP from the tethered mode so your stock recovery will be intact.
3. SuperSu initial installation will modify some files in your boot partition in order to re-direct the system request to the SuperSu, as such In case you want to receive an OTA file later, do
**REMEMBER to FLASH BACK TO STOCK Android 6.0 BOOT.IMG** to unroot before upgrading to official Android 6.0.
4. Make sure your phone has battery 80% or more.
1. USB drivers for Asus have to be installed in the computer and your computer should recognize and can do the file transfers between the computer and your phone. You can get the drivers here. You will need to allow the file transfer from your phone when prompt.
2. Android debug bridge (ADB) has to be installed in your computer. I used ADB version 1.0.32. Some knowledge in using DOS and ADB is a plus.
3. Your phone should have the developer option enable and usb debugging should be checked in the developer option tab.
4. You need to know how to use the TWRP. There are a couple videos on youtube explaining about using TWRP.
The Files you need for the root.
1. TWRP - you can download the latest twrp file from here. The latest is ver 3.0.20. You could use older 2.8 versions if you want it.
2. SuperSu installer - I did try with latest SuperSu without success as such use the SuperSu patched by @shakalaca . Get the BETA-SuperSU-v2.74-2-20160519174328-patch-by-shakalaca-for-zenfone2-6.0.zip file here.
1. Copy the downloaded TWRP to the directory that you install the ADB. (If you do not want to type long file name in adb, rename it to twrp.img. )
2. Connect your phone to the computer and copy the SuperSu installer to the phone internal memory or the external SD card.
3. Open the command prompt windows with administrative rights. (Without admin right ADB will not work properly).
4. Go to the ADB directory using the cd command. (e.g. cd\, cd adb, etc...)
5. Start the ADB environment by typing "adb start-server" . (without the quotation marks of course). You should see ****the demon started successfully*** message.
6. If you want to check your phone connection in adb, you can type "adb devices" to see your phone is recognized in ADB environment. You should see your phone serial no. and status .
7. Boot the phone to bootloader by typing "adb reboot-bootloader". The phone should vibrate and go into the fastboot mode. Wait till your phone get in the fastboot mode before you proceed.
8. Give the command to boot the phone to the twrp by typing "fastboot boot twrp.img". (If you did not change the name of the image file you need to type or copy and paste the whole file name).
You will see the message saying like-
"downloading 'boot.img' .....
finished. total time: 1.120s "
The phone will vibrate twice and the Asus Logo with black splash screen will appear. The phone is start booting with TWRP image.
9. Unplug the phone from the computer. (If within 30 sec your phone should not start booting to TWRP. you can press and release the power button to go to the normal boot mode or press and release the volume-up key to go to the recovery mode. The TWRP should start by then.)
10. In TWRP install the SuperSu file by tapping install option. (I am sure you remember where you put the SuperSU installer zip file)
11. When the SuperSu installation is completed you will see the message saying like " You will see the your phone has failed verification message and reboot a couple times". Please remember that.
12. Clear the Dalvik cache and then reboot the phone. (You can reboot the phone without clearing the dalvik cache if you want).
Your phone will reboot and you will see the triangle saying like "your phone has failed verification" message and then automatically reboot the phone, showing up the message again. It would happen two to three times and you phone will be booted to the system. (You will not see that error message anymore in the upcoming boots).
Now your phone is rooted. You can check it by installing and running the app like root-chekcer.