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UPDATE: THIS ONLY WORKS IF YOU HAVE NEVER PLAYED ONLINE.

Win a match with either Joker or Bane (they also get a slight amount for the over all XP). I've also already one with each faction on every map so possible trophies include

Killing Joke (play a match with Joker team)
Fallen Knight (play a match with Bane team)
Gotham All-Star (play a match in general)
Arsenal, Awesome (play with one of the guns already at level 9)
Tales of Gotham (win just once on any map)

Then finally go into loadouts, select Prestige and that should give you Legend

Again, these are untested. SO 50-50 shot if they'll work. If they do, it will save you countless hours and migraines of this horrid multiplayer.