This is an endurance match: you have to fight until you're defeated or until you manage to defeat all 100 opponents (1 point for each enemy defeated). Medals are won after you score a specific amount of points (like in standard combat challenges).
Here's a "100 to 1" combat challenge I found on Youtube, made by a guy owning a channel called "BatmanArkhamVideos": check this video out, this guy is a real master with Slade, I'm waiting to see him playing as Batman! Plus, he also made awesome walkthroughs for the first two titles of the Batman: Arkham franchise...
here's the link to the video - http://www.youtube.com/watch?v=fubOePJ-d4o
I think the Earth 2 Dark Knight has to do with the following explaination... Arkham Origins iPad or iPhone app gives you this skin as you defeat the Tier 1 section. You must have a WBID and be linked to your console.
Hope it proves useful, although I prefer the new 52 and the "Gotham by gaslight" skins...
I also think there's been that little accident involving Bane breaking Batman's back: think about the Bane boss fight in Arkham Asylum, especially after Bane says "I will break you Batman, the bruja!" and Batman replies "No Bane! This time I break you!", could it be a reference to the Knightfall storyline?