Quake

XBOX FOR PC PLAYERS ONLY:
First you need to be on nightmare mode. Here is a video that shows you how to access nightmare mode for all the campaigns. But for this solution just pay attention to the beginning of the video:
Hit the ~ key and first type "map e4m6" (please note level select seems to nullify some achievements but using the command code work with no problems)

Next enter the "god" mode cheap

Lastly, you can type "noclip"

I found the easiest way to find the exists is to usually circle just outside the map, you can see a lot of the map this way. You are looking for an archway. Find it, enter "noclip" again to turn it off and walk through the archway exist.

(If you are going to downvote please leave a comment, a better solution, or a reason why you think this is an improper solution. I can not change the fact the console players can not use cheats)

IF GOD MODE ISNT WORKING: (all credit goes to Chocotoaco119)
According to a recent post on reddit you need to type "sv_cheats 1" hit enter and then type "god". You should get a notif in the console that god mode is on.
MAKE SURE YOU TYPE "sv_cheats 0" AFTER YOU HAVE ACTIVATED GOD MODE OTHERWISE YOU WILL NULL THE ACHIEVEMENT. God mode will still stay active after entering

If you're on console, you won't be able to use cheats to help you on getting this achievement, so get ready for some grinding.

As soon as the level begins, grab the Quad Damage, get down and kill the three fiends roaming the area. Go to the door towards your right and shoot the two fiends in there, but don't go in or'll you attract some spawn (unless you're feeling confident, but time is critical here). Now head the other way, kill the vore, grab the grenade launcher and step into the room behind the sliding bars. This room has a moving wall that'll push you towards a dungeon filled with spawn. Aim up and start firing grenades. Spam a lot, then pull the nailgun and finish off any spawn still lurking in the area. Remember to check your health from time to time to guarantee you didn't take any damage. The rest of the level can be completed by save-scumming without much hassle.

Other tips:
- Remember to save often
- There's a secret wall underwater you can shoot in the area with three hell knights on beams that will reveal a pentagram of protection. You can use it to have an easier time clearing out this room (thanks Karachi King for confirming it works)
- Don't linger too long underwater or you'll start drowning and void the achievement.
- Some areas get repopulated with enemies once you clear them out and move away. Always be careful when entering a previously cleared room.
- Finally, don't sweat it if you're already past this level or playing the episodes out of order. Once you finish the game, you'll unlock a Level Select function that'll allow you to retry this as many times as you want. I'd recommend jumping to either the level before, or the secret level before you attempt this, as you'll otherwise start the level with just the shotgun (thanks Hengeste for pointing this out).

Easiest way to get this on Xbox One is, if you have a second controller, do this in multiplayer coop splitscreen. The player who gets the achievement can remain idle. The second player can play through the level and die and respawn as many times as they want. Only watch out when you grab the gold key. An enemy spawns right in front of the idle player. Make you sure that you run forward with your active player after you get up the elevator. You will drop down into the first area and the enemy will follow you and won't attack the idle player.
Splitscreen also works for the Slipgate Pacifist achievement.

This guide will show 2 ways to get this achievement: one will be with level select, and an easier (probably) way that will require you to go earlier in E4 to grab the Grenade Launcher for a Grenade Jump.

LEVEL SELECT METHOD: Grab the Quad Damage and use it to kill the Fiends in the room. I would recommend positioning yourself right below where the Quad Damage is located after you have damaged one or two of the Fiends, you should be able to safely kill them in that spot. Before you move on, make sure to flip a secret switch located on the lower left column in the room. You don’t need to do it now, but it will make the return trip a lot easier. Run left and past the Vore till you get to a room filled with Spawn. You’ll need to kill most of them before the moving wall behind you pushes you off, as it will be basically impossible to get out of there unscathed otherwise. When you eventually (it is very unlikely you’ll get this first try) manage to take them all out unscathed, grab the Gold Key and head up the elevator; make sure to position yourself on the left side to avoid getting hit by a Spawn that is now at the entrance. If you hit the switch ahead of time, head to the right to find a secret route to the Silver Key; if you didn’t, hit it, evade the Spawn while you wait for the route to reveal itself, and head to the secret route.
Move to one of the sides (I would also recommend jumping as well) to avoid getting hit by a Fiend and do the same for the 2 Death Knights. When you get the notification that the Unholy Altar is defiled, grab the Silver Key immediately and make a run for it. When you turn right, briefly take refuge at the nearest corner to avoid getting blown up by an attack from one of the Vores. Dive into the water and take out any Rotfish that are in your way. You’ll come to a room that has a Biosuit, Ring of Shadows, and a secret wall* close by (revealed by shooting it) that contains a Pentagram of Protection; note that damage taken while under the effects of a Pentagram of Protection does not void the achievement. Take the Pentagram and immediately afterwards the Ring and use the Golden Key to activate the lift that may or may not have a Death Knight standing on it. If it does, shoot him until he dies; if it doesn’t, head to the Silver Key door that is probably behind you. Note that from this point on, aside from the second Unholy Altar, you do not need to (and should not) shoot anything.
Activate the switch in the next room and pick up the second Ring of Shadows on your way up the stairs. After returning to the previous room, jump onto the nearest beam, then jump onto the platform right next to the one you jumped off from. Swim through the water, shoot the Unholy Altar till it is noted as defiled, and run to the exit.

Video guide by I’m sleepy maybe.:
GRENADE JUMP METHOD: Kill the Fiends with Quad Damage and head straight to the right of the room where you’ll find the water; jump into it immediately. When you make it to the next room, grab the Ring of Shadows and Pentagram of Protection for the Grenade Jump. Head opposite the side of the Golden Key button and shoot a Grenade low onto the ground. When the Grenade is about to explode, move forward. If you did it right, you should be on the platform that’s connected to the last piece of water in the map. Swim through the water, shoot the Unholy Altar till it’s defiled, and run to the exit.

Video guide by blobs: *Second video guide contains the location of the secret wall.