Hard Reset to Factory Settings without USB access
I completely forgot you don't need debugging and all thing ....... On windows/mac/Linux
1) Just open internet browser and go to click herewww.google.com/android/find
2) Enter your email id and password(which one used in asus zenpad z300c)
3) If you have multiple device with same id then selected Asus zenpad z300c carefully.
4) If only one device is register then skip step 3. And In left hand side select option ERASE DEVICE .
5) Might be Google will confirm password again. So enter it again.
6) Done.
7) Your device is factory reset.

note **Your device Asus Zenpad z300c need to connected to internet to perform this operation prefer wifi connection **
Good Luck.....

Please update it is successful or not.