Falling For You

Knock an enemy off a ledge whilst fighting them. There are plenty of opportunities for this through the game, but the easiest is probably the Hall of Valor arena - which can be unlocked for a side quest by talking to Taash at the Lighthouse. The big hole in the middle of the arena counts as a ledge, so its easy to push people in regardless of your character class.

Or, as a Fighter, the first two chapters have plenty of cliff edges, and a nice hit with your alternate/heavy weapon will easily send someone flying off the edge for an easy unlock.