If Asus hasn't coded it so it can't be rooted, connecting it to a PC or laptop and download KingoRoot and install it on to a Windows system. Next, you need to set the phone to debug USB connections.
To do that, you need to look in Settings>More>About Device and tap Buld Number seven times to become a Developer.
Developer options then shows up in the menu and inside that is the tick box named USB Debugging. Tick that and connect to the laptop.
Start KingoRoot and see if it works. If it fails, try another root utilily from the Play Store. If that fails, it can't be done and it's pointless going through lots of other possibilities. It can upset the system..