The Halo universe is largely inspired by a variety of works within the genre of science fiction, and the Alien/Predator universe is no exception. But let's say the Chief needs to take care of a few Aliens himself? I'll refrain from limiting it to a set number at first, and we'll go from there.

Round 1: Corbulo Military Academy is under attack. Students, instructors, and MP officers are being systematically slaughtered as they attempt to evacuate. The space elevator has been destroyed, and every nearby craft has been crashed as they attempted to take off. Only a handful of survivors hide, paralyzed in fear, when suddenly... Can Chief save them?

For this scenario, John has minimal experience as a Spartan, given that he's only 15 years old (I know right!?). His Mk.IV armor is significantly weaker than his current set, and does not emit energy shielding. At his disposal is of course an MA5B assault rifle (60 1 per magazine, 7.62x51mm FMJ-AP) with 5 mags of ammunition, and as a sidearm, an M6D Magnum pistol (12 1 per mag, 12.7x40mm SAP-HE) with 6 additional magazines. On his person is a pair of M9 fragmentation grenades, as well as his issued combat knife, which is capable of piercing through Covenant armored bulkheads. His mission is to safely evac all survivors, the 4 cadets in the clip specifically. While they have nearly completed training and are certainly talented, they have no combat experience and require Chief's guide. Outside is an M12 LRV "Warthog" in which they plan to make their escape to the evac point, where Spartans Frederic-104 and Kelly-87 are awaiting their arrival with a Pelican gunship, both with the same loadouts. However, it's still not that simple. All the while, a Predator(s?) is hunting them, curious to add this strange human warrior's vertebrae to his collection.

Round 2: The Pillar of Autumn has been forced into a "random" (sure Cortana) jump the by Covenant after the destruction of Reach, and as they exit Slipspace, before them lies the ominous and awe inspiring ring construct called simply: Halo. On this mysterious ringworld contains ancient evils unlike mankind has ever seen before (or have they?). Can the Master Chief stop galactic genocide here, before it's too late?

For the most part, Chief's equipment is largely the same except for his armor, the Mk.V, which is significantly stronger than his earlier set, as well as providing shield generators capable of deflecting enormous amounts of damage, provided that it doesn't interrupt the electromagnetic field. It's a frictionless surface that will resist all forms of matter, including a particularly annoying liquid substance. Cortana is also present to assist Chief with both micromanaging him in combat and maintaining constant awareness of the ongoing battle by accessing friendly and enemy comms, and can relay information between Chief and fellow UNSC forces. Chief here is also thoroughly experienced in combat, having fought the Covenant over 26 years of constant warfare. As for other UNSC forces and supplies, the survivors of the PoA's assault have captured a Covenant outpost on the ringworld dubbed "Alpha Base", and have stockpiled a collection of both UNSC and Covenant equipment, including vehicles, ammunition, weapons, turrets, etc. The majority of their ground forces are also made up some of among the UNSC's finest, the ODSTs (yes, this is all canon from the book "The Flood"). Chief's mission mirrors that of the first game, with the exception of the ancient threat being composed entirely of Xenomorphs as opposed to the Flood. A ship of Predators have also tailed the Covenant fleet and plan to take advantage of the ongoing carnage to hunt the various species on the ring. They will side with the humans after the Xenomorphs are released however. Do things end differently as a result? Who survives?

Round 3: This is it. Time to end the reign of terror spread by this pestilent race of Xenos. Chief and the Arbiter set out to destroy High Charity, and the Xenomorph Queen altogether, who hides in Installation 04B's Control Room. First, Chief must save Cortana alone, who is trapped in the heart of the ship, escape with the Arbiter in the Pelican, spark the ring, and get to the UNSC frigate Forward Unto Dawn before it's too late. Can they do it?

Chief is wearing Mk.VI armor, which you guessed it, is even stronger than Mk.V in every way. It also has the added feature of being able to heal minor injuries instantly using biofoam. He starts out with only an M90A shotgun and Plasma Rifle, but can scavenge for weapons onboard. Arbiter has his personal Energy Sword, dual Plasma Rifles, and a Covenant Carbine for distance, as well as a decent number of plasma grenades. His combat harness also emits energy shielding, although they are marginally weaker than Chief's. He can also cloak, but only for a limited time before the active camouflage unit must recharge. Sergeant Major Johnson will appear when the duo reach the outer gate to the Control Room, armed with a Spartan Laser. 343 Guilty Spark and his Sentinels will provide limited support with their directed energy weapons, but due to the infancy of the ring, their numbers are scarce and susceptible to attacks from Xenos (their shields only defend from projectiles).

Was thinking of doing a 4th round, but I have to go to work soon, so fuck that.

*Side note, if you can, make the Queen kill Guilty Spark in a hilarious fashion before he zaps Johnson. That way they might all make it out alive that way :,)