In the world of XCOM: Enemy Unknown, even the lowliest of alien adversaries can be lethal. Especially early on. Whether it's a Thin Man's poison or a Heavy Floater's mobility, every single species has at least one kind of combat specialization you'll want to look out for. Simply shooting at something until it's dead can certainly work, but if you want to kill it and keep your squad alive, you should probably take a look at the tips below.
[SPOILER WARNING: This guide lists all of the aliens you'll encounter on missions throughout the game. If you don't want to be surprised, please stop reading now.]
Sectoid - Sectoids are one of the most fragile aliens you’ll encounter, but that’s no reason to take the lightly. They’re numerous, they’re quick, and they can boost each others’ combat abilities by linking up telepathically. Of course if the Sectoid that initiated the mental link is killed, both will die. Just stay in cover and pick them off and you should be fine.
Floater - Dealing with Floaters can be a bit tricky because of their mobility. They can simply fly up to “the high ground” whenever they feel like it for an accuracy and defense bonus, and can even airdrop themselves anywhere on the map at the cost of their turn. When one or more Floaters goes missing, it’s a safe bet that they’ve repositioned themselves and are planning to flank you, so make sure to keep everyone in Overwatch whenever possible.
Thin Man - Thin Men are about as formidable as Sectoids (i.e. not very), but they’re far more mobile due to their ability to leap over large walls or on top of buildings. They’re also poisonous and can use that to their advantage by spitting venom at soldiers or leaving a noxious cloud behind when they’re killed. A Thin Man’s poison doesn’t do much damage, and it only lasts a few turns, but it can add up so try to stay away from those green clouds.
Muton - When you first encounter Mutons, you know things are getting serious. These green jumpsuit-wearing aliens are essentially the enemy equivalent of a typical XCOM soldier. They’re formidable, come equipped with Plasma Rifles, are a lot more accurate than most of the other aliens you’ll have encountered up to this point, can use their Blood Call ability to boost their allies’ offensive capabilities, and have alien grenades that they aren’t afraid to use. Mutons are Enemy Unknown’s first real test, but things will only get tougher from here.
Chryssalid - I mentioned Chryssalids in the beginners guide but the warning bears repeating: do NOT let them get close. Chryssalids are primarily deployed at Terror Sites and will often go straight for any civilians they find. When a Chryssalid kills a human (including your own soldiers), which is very easy because their razor-sharp claws to a lot of damage, they also implant a sort of egg into them. This will create a sort of zombie - which is also a rather durable and hard-hitting enemy - that will shuffle around for a few turns before bursting open as a new Chryssalid jumps out. Yes, they reproduce. They’re incredibly fast, do lots of damage, and create more of themselves by killing things. Never, ever, ever, ever let them get in close to your soldiers.
Cyberdisc + Drones - The Cyberdisc is basically the aliens’ equivalent of a light tank. It’s rather mobile in its disc form, and a horribly intimidating death machine when it changes into its combat form. It can lob grenades, fire off some really heavy hitting plasma weaponry, and is usually flanked by two drones that can also deal a bit of damage with some small lasers and repair damage that the Cyberdisc has suffered. While it might be tempting to take out the drones first so they can’t fix anything, the Cyberdisc is a far bigger threat that can usually be dealt with in one turn provided enough soldiers are close by and can perform actions when it appears. Another benefit is that the Cyberdisc is too big to use cover, although it does still benefit from elevation bonuses. Also, try to keep clear when you take a Cyberdisc down as they explode afterwards.
Berserker - Berserkers are a very intimidating breed of Muton which usually appear with two regular Mutons in tow. They don’t bother with fancy weapons, but rather bum rush anything they don’t like and try to rip it limb-from-limb. Much like Chryssalids they make up for their lack of ranged attacks by being more mobile, although Berserkers aren’t quite as speedy. The flipside is that they take a free movement action towards their attacker whenever they take damage. That, and they can Intimidate nearby soldiers, causing them to panic and lose a turn. This can be advantageous, however, as it’s possible to corral a Berserker over the course of a turn by taking pot shots at it with several soldiers while drawing it into range of the bigger guns.
Heavy Floater - All of the mobility, elevation, and air-dropping benefits regular Floaters possess are also a part of the Heavy Floater’s skillset. However the Heavy Floater is far more durable, uses stronger weapons, tends to be more accurate, and makes liberal use of grenades when possible. The same basic tactics for Floaters apply, but doubly so.
Muton Elite - Muton Elites are essentially badass Mutons. They’re tougher, hit harder, and fight harder than the average Muton. They wield Heavy Plasma guns instead of the more run of the mill Plasma Rifles, and like to use grenades a lot more. Oh, and they also like to show up in groups of three. Best to treat Elites as you would regular Mutons, but be a bit more cautious and don’t hesitate to bring out the big guns.
Sectopod + Drones - Whereas the Cyberdisc is the alien equivalent of a light tank, the Sectopod is more like a full on battleship. This mechanized walker fires a devastating laser (twice!) as well as a cluster of rockets, and it has the most health out of any regular enemy unit in the game. Oh, and it also has two damage-repairing drones with it. Sectopods are pretty much the entire reason for teaching a heavy soldier the Heat Ammo skill since it gives them a bonus 100% damage against robotic enemies. Assuming a heavy soldier is present and in range, a rocket certainly wouldn’t be out of the question. At the very least it should destroy the drones and damage the primary target. A sniper with Disabling Shot is also handy since they can use it to shut down the Sectopod’s primary weapon for a few turns and give the squad a chance to regroup.
Sectoid Commander - Think “regular Sectoid,” but on mental steroids. Sectoid Commanders are only slightly more sturdy than their underlings, but they make up for it with some pretty serous Psionic powers that go well beyond simple being able to buff their pals. Mind Control isn’t out of the question, actually, so it’s imperative to make Sectoid Commanders a top priority target, unless an Ethereal is present. Kill it and the controlled soldier will be free, but if not you’re going to have one heck of a serious problem on your hands.
Ethereal - These incredibly powerful Psionic aliens have no need for weapons. Their minds are weapons. Taking on an Ethereal is no easy task as it’s packed with offensive and defensive skills. It puts up a Psionic shield that makes it very difficult to hit, can prevent damage when it is hit, has a tendency to counter-attack, and has an entire arsenal of mental attacks at its disposal. If handled improperly, an Etheral can all but wipe out an entire squad by using mind control, causing panic, and simply tearing a target’s mind apart. The two Muton Elite escorts are just gravy. Thankfully mechanical devices such as the S.H.I.V. are totally immune to mental attacks and it’s possible to develop Mind Shields for your soldiers to equip in order to boost their defense against mental attacks. Treat with extreme caution, regardless. There’s absolutely nothing wrong with a little overkill.