Kingdom Hearts II
When investigating the Seven Wonders of Twilight Town, Roxas checks up on the "Shadowy Doppelgänger" myth and ends up in front of a waterfall. He sees a reflection of himself, which turns into a dark Roxas clone. It then engages in battle with him. Shadow Roxas is last seen after Roxas and friends leave the waterfall, peering out ominously until dissipating into nothingness.
Shadow Roxas is very easy in the fact that he only has one ability on him and that is Combo Boost. He uses a Struggle Bat against Roxas. The simplest method is to Guard or deflect his attacks, then hack at him. However, he does not give any experience once he is defeated.