After a couple of more difficult story missions, Episode 8 slows the pace and offers a much shorter, easier mission. For this episode, Musselback (the gun/drill arm) is not available. By the same token, you can't use dodge burst or adrenaline burst, so if you've relied on those up until now, you're out of luck.
Fortunately, there are only a few enemies to encounter as you walk the path and reach new areas, and they're all the smaller, standard kind.
The episode is mostly about the cut-scenes. In the "boss battle" there are several enemies ahead of you in a semicircle. Quickly identify which one is David, aim at him by using the left stick, and press X. If you don't do it fast enough, he'll come attack you, but all in all, it's pretty easy to do. Do this a few times (around 4 or 5 times, if I recall) to finish the mission. A slightly different version of a prior cut-scene will play out, followed by a scene with Bryan, and then the mission results (where the achievement unlocks for you).