In terms of AP, Jimmy got them beaten, but in this match, he and the alien's AP are mostly even (even Jimmy's weapons, as I've said in my comment above). In terms of abilities and skills, I doubt I need to explain Robocop and Terminator's marksmanship or skills to you because those two completely outclassed a teenage boy to an unimaginable degree.
That is not to mention their sensory abilities, like enhanced senses, information analysis, etc. None of these aren't enough to beat the creature.
Alright then, let me see if that is true:
- The Thing is afraid of fire because it basically negates their regeneration, which is why Jimmy's firey weapons are a huge deal in that match. Xenomorph resist fire and high temperatures, plus they have pain tolerance + comparable durability to survive them.
- The Thing possessed human bodies in that match, so itching powder and stink bombs still work. Xenomorph's body is covered in this exoskeletal structure, the powder can't make them itchy. It lacks eyes and senses everything in different ways than humans, stink bombs won't be able to stun them.
- Xenomorph was able to navigate through facilities, tight spaces, spaceships, caves, or whatever locations are being used in films, games, and novels and use things like ventilation systems or blind spots to ambush its prey. Bonus points for its ability to crawl on ceilings and walls, which makes Bullworth an ideal ground for it to play the stealth game.
- The Thing outperformed the Xenomorph in terms of intellect, but the true difference is how their intellect feats are applied in certain areas: the Xenomorph's intelligence is demonstrated by their hunting skills, whereas the Thing's highest level of intelligence is based on their engineering skills (they manage to build a spaceship from scrap) and manipulation skills (faking acts and deceiving people). One side's superior intelligence does not guarantee a chance to win unless they can actually capitalize on it during the match.
- Finally, Jimmy is unaware that Xenomorph's blood contains acid. While in the fight with The Thing, the OP let Jimmy know not to touch any of The Things, he would be unaware of the acid blood until the Xenomorph bled or attempted to spit acid at him.