After you've completed Proto's sidequest, head back into the movie-theater (you may need to rest at medmay first). Find Proto sitting in the car on the ramp watching a movie, talk to him and offer to teach him some moves.
He'll start honking the car horn, attracting waves of enemies.
Fight the hell out of the 25 waves.
This will be tough.
I suggest usuing an energy based build, use implants that will give you buffs for filling in batteries and for executing enemies. There will be a lot of hits and a lot of executions. (Blocking seems kind of pointless here)
In the middle of the fight, he'll ask you if you need a break. Talk to him and tell him you want to continue!