A quick guide to get your ASUS WL-520GC back to factory firmware if you have a bad flash (AKA Bricked).
You will need;
1. ASUS Firmware utility
2. Original firmware
You can find both of the above from the ASUS website or simply download from links below
As always proceed at your own risk
First we need to get the ASUS into recovery mode, do this by holding the reset button for 3-5 seconds, the power button will begin to blink slowly (2 seconds on 2 seconds off)
Connect your computer directly to the ASUS router with a LAN cable
Next we need to give our LAN a static IP address on the devices range (Default IP of the ASUS is 192.168.1.1) so we will set a IP address of our pc to something in the same range. Test if you can ping 192.168.1.1
Extract FW_WL520gc_2010_EN.zip to your desktop so it is easy to locate
Now we need to extract the contents of UT_WL_520GC_3540.zip and run the setup wizard. Once installed run the ASUS Firmware Restoration tool
Locate the firmware you extract to your desktop,select it, hit upload. Wait for the tool to finish and you should be back to original firmware.