As this guide notes, right now you can just use sv_cheats 1, toggle "god", and get the achievement that way. But in case the devs ever patch this (or you want to earn it more legitly), this guide will detail how to do it without cheats and still pretty easily. It's assuming you have no save files prepped or experience playing Nightmare, but it does assume you know how to grenade jump.

(Frankly I also think this method is quicker and more fun than playing through Pain Maze with god on)

The only weapon you'll need is the grenade launcher. (If you happen to already have a Pain Maze save file on Nightmare with a grenade launcher, skip to step 3.)

Start up "Palace of Hate" on Nightmare and follow this speedrun path. The only hard part is the grenade jump over the water, but you can use a quicksave right beforehand to get infinite tries:

• Run down hallway, sidestep the Fiend (don't miss the super shotgun, it'll help in Step 3).
  
• Take the left way forward, then turn into the right archway to reach the healing spring and pick up the grenade launcher
  
• Bypass the knights and head straight (Watch your ammo if you use any grenades for self-defense, you barely have enough to make all the upcoming jumps)
  
• You can go left or right around this next bend, either way sidestep the Shambler and reach the water
  
• Cross the water, then grenade-jump from the alcove over the water and into the opposite room (recommend quicksaving before you jump!)
  
• Once across, grab the silver key and the Pentagram of Protection, then grenade jump up the secret portal
  
• Drop down onto the Shambler, do a 180-turn and run straight to the silver door. Go through it to reach the exit

The next map, Hell's Atrium, can be finished very quickly and without fighting anything. The only tough part is the final jump over the lava pit, but again you can use quicksaves to get infinite tries:

• Sidestep zombie, run straight through courtyard
  
• Grab Ring of Shadows, continue running straight
  
• Run through the room, jump over the lava pit by landing on the tiny spikes (recommend saving first!)

Now that we're actually in the proper map and with the proper weapons, we need to be a bit more careful. But it's still pretty easy if you've played any amount of Quake.

• Grab the quad damage at spawn, blast the Fiend out of your way with your super shotgun, take the right door and drop down into the water passage
  
• Shoot Rotfish out of your way, head to the central chamber of the map. Swim to the ring of shadows and collect it
  
• Shoot the underwater green-colored square on the central pedestal to open a secret, grab the Pentagram of Protection inside
  
• Jump onto the pedestal, angle yourself so you're facing the exit, grenade jump up there (quad-damage must still be active for the extra boost)
  
• Sprint past everything, they won't see you because the ring of shadows. Shoot the altar enough to defile it before navigating to the exit.

And there you go! This is the fastest way to easily get this achievement, far as I know.