Enable USB debugging mode.
Install LG USB Drivers
How To Root LG G Flex
Connect your LG G Pro 2 with PC.
Unzip IOroot file and open the extracted folder.
Right click the Run.bat file and choose run as admin, For Mac choose root-mac.sh and for Linux chooseroot-linux.sh
A cmd window will appear and all you have to do is to press any Key
In only a matter of Seconds, your LG G Flex will be rooted.