This is a list of the enemies in Halo: Combat Evolved.
Covenant[edit]
Unggoy (Grunts)[edit]
Cannon fodder for the Covenant, they are the smallest and weakest but most numerous Covenant warriors. They wear methane tanks that allow them to breathe. The minors have orange tanks with plasma pistols, while the majors have red ones with needlers. Occasionally, a Grunt may be seen with a fuel rod gun. Don't waste too much precious ammo on them, take them down with a pistol, or more preferably melee them. You can also leave them to your Marine comrades. Be especially wary of the fuel rod gun Grunts. To combat them, you may want to move up closer to them so they don't fire because of collateral damage.
Sangheili (Elites)[edit]
The ironheart of the Covenant, they are relatively strong and are a common enemy. They have energy shields very similar to yours which you have to deplete before dealing any real damage. Almost all of them carry plasma rifles. They come in many flavors. Blue-armored ones are the easiest, as a burst from the assault rifle followed by a melee often kills it. Crimson Elites are harder, but not by too much. Invisible ones are, what else, almost invisible, although carry no shields and thus you can just spray the assault rifle wildly in the air. Golden ones are very tough and most of the time, they will carry an energy sword that cuts through Marines like butter (and you). However, they usually have no ranged weapon, so just shoot from afar. These will always be your biggest threat.
Finally, on the final level, The Maw, be wary of Special-Ops Elites. Clad in jet black armor, they are stronger than all but Golden Zealot elites. It is worth noting that not only are their shields stronger, they also have a unique ability - they throw their plasma grenades at over twice the speed of other elites, resulting in an unexpected death for the foolish.
Kig-Yar (Jackals)[edit]
These avian creatures carry large round shield that, unlike the Elites, have a physical form and are thus avoidable. However, the shields are indestructible to human ballistic (bullet) weaponry and only destroyed by plasma weaponry or melee (charged plasma pistol shot removes the shield instantly). They all carry plasma pistols and have only two ranks. The blue ones have less health than the red ones. Otherwise, just don't let them charge their pistols, because your shields go down like that!
Grenades are very effective. Otherwise, shoot their exposed hand and then kill it with a pistol headshot as it recoils.
Lekgolo (Hunters)[edit]
These hunks always appear in pairs and are rather large. Their entire bodies are invulnerable except for two small orange spots. Their weapon of choice is a modified version of Fuel Rod Gun. Despite this, with enough skills, they are fairly easy to kill. Just shoot the orange part with the pistol. Instant kill! Watch out for its melee attacks!
Flood[edit]
Infection Form[edit]
These are the small, popcorn-like things you see often. Despite their swarming tactics, a shot from anything will kill it. They also explode when killed, creating splash damage and a potential chain reaction across the entire swarm.
Although tempting, shotguns are inefficient against the swarms of the infection form, as a single bullet from an assault rifle can do the same amount of damage from one shotgun blast. Shotgun shells are best saved for the combat and carrier forms.
Combat Form[edit]
A Flood that had infected (or took over) a human or an elite. Plasma weaponary is less effective against them. Most effective way to defeat them is to shoot their hearts with a shotgun or 7 rounds of needler. Meleeing is useless, however. They'll occasionally wield weapons. Since they use melée attacks so often, it is unwise to use plasma grenades as you will likely be killed by the resulting explosion when they charge.
Carrier Form[edit]
These giant blobs are just walking incubators. When shot, they explode and about 6 Infection Forms pop out. Don't get close to it; the explosion has rather large splash damage.
Sentinels[edit]
These robots are your allies for a bit, but then turn on you. Simply use a Covenant weapon, preferably overcharged plasma pistol or plasma rifle, and they'll go down quick.