How To update OTA without bootloop on Rooted Zenfone 2

First things first: (get this straight, don't download the wrong firmware and flash the wrong thing)
ZE550ML = Z008
ZE551ML[2gb,4gb ram] = Z00A

Files Needed:

1. Drivers for Asus Zenfone 2 installed on PC
2. ADB installed on your PC (Download)
3. Flash Tools (Download)
4. Pre-rooted system.img and recovery (Download) (Download the correct firmware which is present in your phone)

Steps:

1. Download the pre-rooted system.img folder (Download the correct firmware according to your phone ZE550ML = Z008 , ZE551ML = Z00A )
2. Download and run WinRAR . Click on the '.zip' part of the system.img file. You will see that WinRAR puts all the parts together and offers you an option to extract the one big system.img. Go ahead and extract it to the folder the other parts of the ROM are in.
3. Unzip the FlashTools.zip and copy the following 4 downloaded files and paste it into extracted flashtools folder:
   boot.img (~ 11MB)
   droidboot.img (~ 13MB)
   recovery.img (~ 11MB)
   system.img (~ 2GB)
4. Disconnect your phone from PC/Laptop.. Power it off. Once it is off, hold down the volume up button and hold on to the power button until the phone vibrates. After the vibration stops, release the power button and continue pressing the volume up button until the green Android robot appears. You should see a blue line below saying "Waiting Fastboot command"
5. Now connect your phone via USB to your computer.
6. At the flashtools folder press Shift + Right Click.. Then select "Open command here"... 
7. Copy and paste the following commands in the command prompt window (line by line) and press enter after each line. WAIT for the process to complete before copying and pasting the next line:
   fastboot flash fastboot droidboot.img
   fastboot flash recovery recovery.img
   fastboot flash boot boot.img
   fastboot flash system system.img
8. The flashing of system.img will take huge time to show any text after max size info,after some more time it will show finished ,then you can plug out the USB and press the power button, reboot your phone and when it starts up, your phone is ROOTED.
9. Now open Supersu[it will ask for su binary update,click NO],and click unroot from options of supersu,reboot once
   now download the OTA file from here [get the latest one ,check the version number]
10. rename the OTA zip which you download to "MOFD_SDUPDATE.zip", paste the renamed zip in internal memory and external memory[yes paste in both place,i tried to place in internal didnt work ,so try in both]
11. now power off, Once it is off, hold down the volume up button and hold on to the power button until the phone vibrates. After the vibration stops, release the power button and continue pressing the volume up button until the green Android robot appears. You should see a blue line below saying "Waiting Fastboot command" ,Now here click vol down till you see Recovery mode in top big rectangle box,now click power button,and thats all it will update your phone..

WARNING and Note: dont remove bloatwares or system apps if u wish for easy OTA update[use cwm,twrp for easy rooting]