It's a straight fight, one-on-one. They are in a huge white room, no cover, and they are twenty metres apart. The Astartes has the standard kit of a Marine: frag, krak, bolter, combat knife, power armour. The Tyrannosaur has tiny little arms and 'more teeth than the Osmond family'.
My partner and I were discussing this over tea in town today. We came down on the side of the Astartes, but we weren't convinced. I said I'd throw it out to the community, see what the considered response was.