Instructions
Go to Eterna City, located on the northwest side of the Sinnoh region. Walk to the Pokemon Center to encounter a man to receive the Key Item "Explorer Kit." Press the "X" button to open your menu and select the "Bag" sub-menu. Scroll to the "Key Items" pocket using the directional pad and select the "Explorer Kit."
Save your game when prompted to start your local Nintendo DS wireless connection. The Sinnoh Underground connects using a wireless connection, allowing more than one trainer to explore the underground together using their Nintendo DS consoles. Walk forward to encounter Oreburgh Gym Leader Roark, who explains the basics of the Sinnoh Underground. Head to the nearest sparkling wall and press the "A" button while standing in front of it. Sparkling walls contain different treasures, such as fossils and Pokemon evolutionary stones.
Dig for treasures, using the blue pickaxe or red hammer, uncovering them before the rocky wall collapses. The blue pickaxe digs smaller areas and prolongs the wall's collapse; the red hammer digs larger areas, but makes the wall collapse faster. Continue digging treasure in different walls until you uncover the Armor Fossil. Press the "Start" button to transfer the fossil to your bag through the Underground menu. Select "Treasures" to access your items and choose any item's "Send to Bag" option to transfer individual items to your bag.
Exit the Sinnoh Underground and head to Oreburgh City. Walk to the Oreburgh Mining Museum, located on the northwest side of the city. Inside the museum, you can revive fossils by talking to a man seated at the nearest desk. Press the "A" button to give him the Armor Fossil for revival. Pick up your revived Pokemon after exiting the city for a few hours --- the man will hand you a level 20 Shieldon.