Not a slaughterfest, but a Survivors kind of competition. With real survival instead of ploticing as the goal.
Drop a team of ten from each representative 'type' of Space Marines down to the surface...say... Europa. No team may attempt to contact the other team or even come within 500 km of them. No equipment beyond what they carry in their standard kit + a radio is allowed with them. At any moment, a team can call for evac, but that removes them from the competition. Once half have been eliminated, move them to a new environment, like the surface of Mercury. Last team with at least one living member capable of movement on planetside wins.
Note: cannibalization of own teammates is allowed.
And if it looks like no winner can be found in a reasonable amount of time (a few months/a year), drop the remaining teams on the orbit of Neptune. Again, wait for calls of evac, last team on-site with at least one member still alive wins.