Read the first terminal in Rise, Robot, Rise

This terminal is difficult to miss since you have to pass in front of it in order to be able to open the door to progress.

Pick up a shotgun and SMG in Poor Yorick

Both the shotgun and the SMG are behind secret walls.

The shotgun is in the southeast corner of the map. From spawn, use the stairs in the river to get onto the southern ledge, then head southeast to find this door.




The SMG is found on the upper ledge you can reach after pressing the first button.



After entering the secret room, interact with the green box in the corner to reveal the SMG.



In the map below, the shotgun location is circled in red, and the SMG location is circled in blue. (Map taken from lhowon.org)

Kill an invisible defender with the SPNKR

This requires you to kill one of the invisible s'pht'kr in the level Where are Monsters in Dreams. They can only be damaged with splash damage - projectiles will pass through them.

There's a SPNKR rocket launcher hidden in the Z-shaped hole near where you start. It spawns in after you go a bit past it - don't try to drop in before you hear the teleport sound, or you'll take a bunch of damage from the s'pht'kr and not get a rocket launcher.



Once you have a rocket launcher, find a s'pht'kr that's close to a floor or wall and blow it up.

Cripple the Pfhor warning system

Story achievement. Break all three of the panels in the level Acme Station.

Open the Thing What Kicks doors, then let them shut again

Press the two switches in Thing What Kicks and, instead of pressing them again to prop open the doors they control, let them cycle fully.

Take the wrong exit out of Electric Sheep 2

Once you hit the switch to open the path to progress, you can hit the switch that opened the path in Electric Sheep 1. Take that path and loop back to Where are Monsters in Dreams to get the achievement.

Open all the airlocks on Nah Man He's Close

Story achievement. Unlocks when you hit this switch:

Read all seven Eat the Path terminals in one playthrough

Six of the seven terminals are on the normal path, with the seventh being the secret exit terminal.

The first terminal is right in front of you at the start of the level.



Continuing along the normal path, the second terminal can be found after the first set of lava paths, opposite a rocket launcher.



The next three terminals are hidden in a secret path through the window showing an open landscape.
The path is elevated so you have to rocket jump or use the adjacent stairs to get the necessary height. There is one terminal straight ahead and one on either side of the player.




The sixth terminal is the normal exit terminal. Read it and hit escape to exit the terminal without teleporting.

Backtrack to near where you dropped down earlier and sink to the bottom of the lava. This will teleport you onto another ledge with a view of the ledge you need to get to.



Use the knockback from the Assault Rifle + Grenade Launcher to "fly" back to this ledge and continue backtracking to the start of the level. Then take the secret path to get to the final terminal.

Cut power to the containment unit

Story achievement. Break the wires on both sides of this structure:

Fully unblock the upper path before entering it, without using the secret switch

There are 30 switches you need to hit for this achievement. You can see the upper path from the lower area, so you can follow along it to make things a bit easier. If you miss a switch you can save, check the upper area to determine which one, and then load the save and hit that switch. You'll get the achievement when you hit the last switch.

Complete Classic Marathon Infinity

Complete Classic Marathon Infinity on Total Carnage difficulty

Complete the Vidmaster Challenge on Total Carnage difficulty

This achievement requires that you complete the Vidmaster Challenge Levels on Total Carnage difficulty. These levels are more difficult rehashes of the hardest level from each game in the series (Try Again from M1, If I Had a Rocket Launcher I'd Make Somebody Pay from M2, and You Think You're Big Time? You're Gonna Die Big Time! from MI).

They can be accessed from a secret exit terminal in Aye Mak Sicur. Starting from the central area where you get the first chip, take the southern exit (roughly opposite the save terminal)



Then, take the first left and you'll come to the drop. Without dropping down, interact with the wall opposite you to open a secret door.



The exit terminal is in this room.



The above path, with the terminal circled in red. (Map taken from lhowon.org)



Good Luck!