What kind of 1-v-1 fights do you think would be a good match for the composite xenomorph/praetomorph? We've had Master Chief already and I think the general consensus is that in-character he'd keep his distance and make use of high-explosives and probably need to resort to HAVOK nuking the site from orbit... just to be sure.
I feel like trapping it in a gravity centrifuge, maybe getting tossed around in close quarters a bit, before escaping and turning the centrifuge on full power could also work - crush it into a micro black hole that rapidly dissipates through the force of gravity - using one of the four fundamental forces of physics, a reference to exploding the Alien in 3 through the force of thermodynamics.