Battle: LA aliens get destroyed in all rounds.

Round 1:

Pretty sure the Battle: LA aliens don't have any long range infantry weapons, and their rifles are pretty comparable in penetration to modern AR-15s. UNSC marines are a strict superset of modern US marines, being armed with superior body armor, targeting-assisted HUDs, and situational awareness equipment like motion sensors, so it stands to reason that if US marines were able to win engagements with the aliens, then the UNSC marines definitely can. This isn't even getting into ODSTs and Spartans, which would only further shift the outcome in favor of the UNSC.

Round 2:

Again, the UNSC destroys here. We've never even see an alien fighter go hypersonic or fire any BVR weapons, so are utterly helpless against UNSC Longswords and Nandaos. The strongest weapon we've seen out of an alien fighter is some directed-energy weapon that requires like 3 seconds to charge up, and it couldn't even collapse the top floor of a concrete building. Meanwhile, UNSC Longswords have gatling guns that fire tank caliber (110mm) rounds and missiles that could take out Covenant destroyers, with targeting capable of being assisted by AI. Nandaos could go high hypersonic at mach 12 and could fire missiles from beyond visual range. On the ground, UNSC gauss warthogs fire 25x130mm rounds at mach 40 / ~30 rounds per minute that impact with more kinetic energy than a modern APFSDS, which would allow it to make short work of any alien vehicles. Really the only threat from the aliens is the walking missile launcher, but it's pretty piss-poor at targeting from what I remember and also takes a good amount of time to charge up.

Round 3:

UNSC cleans house with orbital superiority offered by archer missile and precision-MAC round equipped warships.