Loyal Defender

If you go to Manda Island, and go into the sub-ruins ahead and to the left of the Flutter, make sure there's a Mimic Chest in the hallway in the beginning. If there isn't, keep leaving and going back in until there is. Once there is, the Golden Miroc will appear in a room on the right side of the ruin. Go right and forward, into a door, and go right again, going into a fork in a hallway, going left. The Miroc will be in this room. A few tips - 

Whether you have Class B or A license, a Buster Gun with max attack will take it out after it trips once when you shoot it enough. 

Class B = 5700 Zenny drop

Class A = 8000 Zenny drop

I do NOT recommend Class S, since after getting the Third Key to Bluecher, you should have obtained/be able to buy stronger buster gun parts. My setup was - 

Power Raiser Alpha

Rapid Fire Alpha

Power Blaster Omega

It'll take the Golden Miroc out quickly. I also recommend bringing the Vacuum Arm, fully upgraded, into the ruins to make the Zenny collection easier, as it'll suck up Zenny from a long range. 

IMPORTANT TIP - DO NOT BREAK THE WALL IN THE MIROC'S ROOM! The Golden Miroc WILL likely disappear if it gets too far away from you. The wall being there prevents this!

A few friends and I calculated, and, based on having a max Attack Buster Gun, and Class A license...

If you press circle to turn around and go in and out the doors to fight the Miroc, kill it, leave, and come back in, it takes 3 seconds to go through the doors. It takes about 5 with your Buster Gun setup to kill the Miroc, and suck up the Zenny with the vaccuum. Rinse and repeat for as long/short as you like (it does get boring, but I listened to the MLP Seasons soundtracks to keep me entertained). It'll drop 8000 Zenny each time with Class A license.