Instructions
Go to Eterna City, located west of Celestic Town, east of Eterna Forest and north of Valley Windworks. Enter the house located next to the Pokemon Center.
Talk to the man in the house to receive the Explore Kit. This kit lets you go to the Underground. Select the Explore Kit in your inventory whenever you are outside to travel to the Underground.
Look at the radar on the screen. If you see a gold, flashing area on the radar, go to that area. Once there, tap the screen. Look for gold spots that appear on the cave walls. If you see one, walk up to it and press "A" to start an excavation.
Tap your tool of choice, either the pick-axe or the hammer. Tap the wall anywhere to break a rock and look for treasure. If you break too many empty rocks, the wall eventually gives way and you must start over at a new location. Eventually, you may find an Armor Fossil at random.
Press "X" and select "Go Up" to exit the Underground. Head to Oreburgh City, located east of Jubilife City and south of Valley Windworks.
Enter the Oreburgh Mining Museum. Speak to the scientist inside and hand him the Armor Fossil. Wait a few seconds. The scientist returns to you with the revived Shieldon.