You can create a custom Shadow Link that appears in others's games via StreetPass to challenge them. Your Shadow Link will be worth a Bounty of Rupees that your opponent can win if they defeat you. To create the maximum 999 Rupee Bounty Dark Link you will need the following:
Fully upgraded Master Sword with 4 Master Ores
The Red Tunic from Lorule Castle
Every Heart Piece and Heart Container from all of the dungeons (except for Lorule Castle, which has no Heart Container)
Upgraded Great Spin Attack from collecting all 100 Maiamais
Two Upgraded Weapons equipped (upgraded by Mother Maiamai for returning 10 Maiamai babies)
If you want to make your Shadow Link truly evil, equip a Blue Potion or two to refill his hearts. You won't have the 999 bounty but your Shadow Link will be tougher to defeat.