To get this achievement, you need to "mark" or "tag" all the different types of enemy in the game. You do this by putting your reticle over the enemy, then hitting the social/attention button (T or D-pad Up by default) There's 23, and while there is a way to see which you've already marked now, it's a little complicated so here's a list with notes on how to find and identify them, because sometimes each different equipment set counts as a different enemy for the purposes of this achievement. However, you may notice that there are more than 23 entries here. As best as I can tell, the two varieties of Cultists (basic and Long-Las), and the two varieties of Hormagaunts count as the same tag- if you've seen other guides, you might notice that the adrenal gland gaunts aren't listed on most of them. The updates to Operations have added more enemies, and I don't know how they count for the achievement.

There's now a challenge to tag all the enemies in the game, and it lists 25.

A few people have mentioned that to get the Heldrake and Hive Tyrant tags, you need to tag nothing else throughout the mission or it will bug out and not count.

With the datavault update, you can now check some of your progress.

Behind the arming hall, you can see the research center.


Here you can unlock little blurbs on the enemies and a 3d model for most of them using the challenge currency.

If you've unlocked it, you can then see if it's been scanned.
The two icons in the top right of the viewing window are its classification (minoris, majoris, etc), and the one below it is if you've scanned it already.

However, if you haven't unlocked it, you can't see any of that information.

The various flavors of gaunt, show up from the start of the game.

Termagant - the small Tyranid shooter
Hormagaunt - the small Tyranid melee
Hormagaunt with Adrenal Glands - the small Tyranid melee with glowing red eyes. Pretty quick so can be a little difficult to pick out.

These are the Tyranid Warriors. They're only distinguished by their weapons.

Tyranid Warrior with Bone Sword - has two swords, usually approaches while guarding, holding them in an 'X' in front.
Tyranid Warrior with Lash Whip - has one sword and a whip.
Tyranid Warrior with Devourer - the "standard" shooting warrior. Sends volleys of small green projectiles.
Tyranid Warrior with Venom Cannon - the sniper.
Tyranid Warrior with Barbed Strangler - the one that shoots the "mines" that slow movement and damage you. Can be identified by not having the green line coming out of their gun, and instead having dribbling green goo.

"Special" enemies

Ravener- the snakey one. Has a special introduction in one of the early missions.
Lictor- the stealth one. Has a mini boss fight early on, and in other small setpiece fights.
Zoanthrope- The floaty irritating support one that comes in pairs. You'll see this one too many times.

Bosses

Carnifex - relatively rare, Mechinus Divinitus is the first time you meet one in a boss type fight. Also appears as a mid-mission boss in the Tyranid Operations.
Neurothrope - Boss fight in Servant of the Machine. Also appears as a mid-mission boss in the Tyranid Operations.
Hive Tyrant - Big boss in Operation Decapitation.

Heirophant Bio-Titan - So I have no idea how this counts, because it released long after I got the achievement. However, it's worth noting that you can tag the final boss of Operation 7 when it walks through the battlefield right before the last boss arena. You can't tag him when you're actually fighting him at the end of the mission.
Trygon Prime - I also don't know how this counts, but it was added with Patch 8, in Operation 9, as part of the Trygon update. It doesn't seem like you can tag it until the final boss arena.

The little ones around the Thousand Sons

Cultist - the humans with lasguns.
Cultist with Long-Las - the humans with sniper rifles.
Tzaangor - the small ones with the bird heads. Some have shields, some have chainswords. They all count as one tag.

The Rubric Marines, and a new one.

Rubric Marine with Inferno Boltgun - the "basic" one. Tend to see it more earlier, but they're consistent anywhere you see chaos.
Rubric Marine with Warpflamer - the one with a flamethrower. Easily spotted by the orange flame icon above their backpack.
Tzaangor Enlightened - The flying birdman on a disc. Easily identified by being another aggravating enemy to deal with in Chaos Operations. Was added in the Obelisk Update, and can be seen in any Operation against Chaos.

Special enemies.

Scarab Occult Terminator with Soulreaper Cannon - has the gatling gun and shoots rockets. Fairly regular as a one-per wave mixed in with the rest of the Thousand Sons. Usually the easiest way to spot him is the dodge attack marker popping up.
Scarab Occult Terminator with Power Sword - Like the other terminator, but has a storm bolter and kopesh (the funny Egyptian sword). Less common, but more likely to show up when there's multiple terminators running around.
Lesser Sorcerer- The flying power-armored support Psyker.

Bosses

Helbrute - used to be called the Chaos Dreadnought. Shows up as a mini-boss in the later missions, and as a mid-mission boss in the Chaos Operations
Heldrake - the flying daemon engine. Is the final boss in Operation Reliquary. Also called the helturkey.
Imurah - the final boss. Can be tagged during his reveal, and then again in the final boss fight. It's easier to tag him before the last fight actually starts, during the long walk.

These, like the name says, don't count and can't be tagged.

Rippers, Gargoyles, Spore Mines, Daemonhost, Lord of Change (the big daemon)