You'll have a new objective. Note the waypoint and head back to the arena. The door is on the 'lower' level, so leap down to head into the Arena.
Once inside you need to follow the path around, counter clockwise, dealing with the loaders that appear to take you out, using the vending machines as you pass them. There's not a lot to these things, you've fought plenty in your time. Remember you can knock their limbs off if you are struggling. The chargers can be easily exploited, too, by jumping up onto any of the crates and debris you can see lying around.
After using the vending machines, you've got to retrace your steps and destroy the vending machines. Uh huh. I actually quite like Oatmeal and Raisin... This is as easy as it sounds. More loads will come to attack, but there's nothing special about this mission.
With everything dead, head to the newly waypointed battle board to complete your montage.