For those who want to root Asus Zenfone 2 and use latest firmware without wiping user data
0. Your phone dont need latest firmware
1. Install Intel USB driver for Android Devices (this is a new version, recently released):
https://software.intel.com/en-us/android/articles/intel-usb-driver-for-android-devices
2. Download the latest ADB and unzip everything from it:
http://forum.xda-developers.com/htc-one/general/tool-updated-adb-version-1-0-32-fastboot-t2932160
3. Download the desired pre-rooted firmware from MEGA:
https://mega.co.nz/#F!k4MHiAgL!dVuOKeH3eokcwPSNI79ffw
(the folder you choose should be the same as your firmware on phone.
eg ze550ml international version is z008 ww 2 19 40 23
ze551ml is z00a 2 19 40 22
Download 3 files from the folder: boot.img, recovery.img and system image - the system image is split into multiple files through a .zip archive...so extract it after all the parts are downloaded)
4. Copy the boot.img, recovery.img, system.img into the folder where ADB installed its files.
5. On your PC, go to the ADB folder, hold shift and right mouse button and choose "Open command windows here"
6. Plug your phone to your PC via USB cable
7. Type adb reboot fastboot. Phone will restart in fastboot mode
8. Type fastboot devices . You should see your device afterwards in the command window.
9. Type fastboot flash recovery recovery.img
10. Type fastboot flash boot boot.img
11. Type fastboot flash system system.img - This takes 140 seconds. Wait till it's done.
12. Disconnect USB cable. then click on Normal Boot button on phone screen
When phone restarts, you notice all your apps / settings are still intact.
13. If new OTA update appear, dont update, it may cause bootloop, just visit the mega link for latest version again