Unlock any of the characters via their totem/ buy with iron. Must complete all waves 1-50.
I completed a full run 1-50 as Baird (Ability to buy turrets is very useful), solo (no AI offline) on Reclaimed. Played on beginner, so deaths do not matter; just restart from the current wave. As long as you don't get disconnected from the lobby and all waves are completed, it should pop at the end of the match.