This is a fun part of the game. You will receive this side mission while playing through the main story of the DLC, towards the end.
(Go to TRANSIT ACCESS, follow the signposts to EVALUATION HALL, go straight ahead, levitate up and dash through the glass window, do not go through the large doors in front of you, instead go down the stairs - signpost says TRANSIT ACCESS, go all the was down, take the elevator at the end of the corridor and you're there... interact with the movie camera and you're on your way!)
While riding on top a 'train', or platform on train tracks actually, you need to cleanse a possessed movie camera. The first part is easy, just survive on the platform, kill enemies by launching stuff at them, avoid obstacles.
The challenge comes soon enough - Third Act Villain boss fight. The boss is a flying enemy that will avoid most of the launched projectiles while shooting rockets at you. It took me a few tries until I figured out how to kill him.
His attacks are quite simple actually. You can keep launching stuff at him, missing mostly, but hitting him every once in a while. Just remember to have enough energy for when he starts throwing those rockets. When you see he's about to attack, raise your shield and wait for TWO rockets to hit your shield with a flash. When they do, you're safe to start launching stuff at him again.
You don't need much else, just patience... Don't be greedy, don't let him get you without your shield ready.
I suggest you MAX OUT your health, launch and shield (you can respec your skills for 30K source, if you need to.) Also personal mods - go for VI tier mods, esp. health, launch efficiency and my third personal mod was replenishing health on launch hits. It was very useful here.
I never used my weapon once.
The worst thing about this is that if you die during the boss fight, you have to do it all over again, which gets boring fast. Just sprinting to TRANSIT ACCESS and getting through the first half of the train ride takes a minute or so. Plus the terrible loading time....