The encounter seems better designed for the 16 man than the 8 man.
I'm assuming Xeno was designed to be PUGed. It's very easy to PUG him on 16m HM; I think in all the weeks the event ran, I was only in one group that did not one-shot him on HM. In the early weeks of the event I was in groups dropping this guy with many people in columi and even some tionese. Once people started worrying about it and reading the forums, groups got way more restrictive about gear. I even got kicked from a group for refusing to get on vent to run him once (...???...). On 16m, all of that is just not necessary.
8m is a different story, but that's always been true in all ops. The dps budget is always tight on 8m. Mechanics weaknesses in the group are exposed much faster.