As bad as the last Underworld movie was, I would back Marius here. We already know what little effect bullets have on him. Only way Cap and Bucky can win is via explosives or a shield decap. But Marius should be fast and agile enough to avoid any grenades or shield tosses at range, and is strong enough to manhandle either super soldier up close. Plus, he can regenerate from any non-lethal damage very quickly.
Funny scene from the movie, starts at 3:06.
Marius/David eating headshots for breakfast.