I think it's where the "enclosed" part of the challenge would come into play. I'd demand that it be air tight for the duration of the challenge.

Although a TRex wouldn't be feeling great at our oxygen levels, I'd be surprised if it would be enough to have it drop dead on it's own. I think you'd still need to fatigue the TRex, and doing anything to further deplete the oxygen in the environment would hurt you both... But the TRex proportionality moreso