Instructions
After the battle with Cipher Admin Eldes on Citedark Isle, head back to the nearby PC.
At the PC, leave an extra space in your party. The space will allow Shadow Lugia to join the party in the final battle against Grand Master Greevil since the battles are fought back-to-back.
A Master Ball is highly recommended to catch Shadow Lugia. If you have a Master Ball available, this is a great time to use it.
If the Master Ball has already been used, you will have to weaken Shadow Lugia while defending against his Shadow attacks. Inflicting status ailment's like paralysis or sleep on Lugia can also help during the battle.
When Shadow Lugia's health is less than 10 percent, use as many Ultra Balls as needed to catch it. It may take several tries to catch Lugia, even if it is affected by status ailments.