Optimal Sprinting

• You consume your stamina while sprinting, but it starts recharging as soon as you stop sprinting
• Which means if you tap the sprint button, you'll enter a sprint and lose some stamina, but it will immediately start to recharge
• So by continuously tapping sprint, you can keep moving at a high speed for extended periods of time
• If you bind 2 different keys to sprint (i.e. Shift & F), you can alternate between tapping those, which seems to be the optimal way of doing this

Exit Reload

• By clicking on "Save and Exit" right before you clear a level, you will skip the elevator rising up
• You can click "Save and Exit" while still turning the wheel to exit by using right click. Do this when you're between 3/4 and 1/2 way done turning the wheel to get the fadeout right after you finish turning the wheel
• Saves about 10 seconds all 8 times you can do it in a run

Elevator Tech

• You can briefly view the map while descending/ascending down/up to the levels by jumping on top of the handle
• In Night 4 & 5 you can use this extra height to start the level a bit earlier by walking/jumping off the handle
• When you find the Power Switch, you can hear the exact direction the elevator is, and to a certain extent how far away it is
• Can use this to sort-of triangulate its exact position (can draw a line on the map that points in its direction for later use)
• When approaching the elevator to escape, you can do a sprint jump to reach it, which will let you keep the running speed, without risking running out of stamina prematurely

Breakable Walls

• Has a ~3s timer starting when you first interact with it
• After these ~3 seconds, the next time you interact with it, it will immediately break
• Sometimes you can interact with it, then search the rest of the room, before you come back to instantly break it

Doors

• Can be opened while looking at your map
• If you close doors behind you and Mary spawns behind them, she'll have to pound on it to get past, which will let you know where she is and buy you valuable time to search and get away from her
• The doors may do the "screaming door" jumpscare, which allows you to run past the door without opening it (although you'll probably have to open it later, anyways)
• The doors can sometimes be used to push yourself for more optimal movement, most notably in-between the nights in the hotel when you can't sprint
• If you've already taken a hit, the door-spike-scare may kill you or prolong the damage, this can be avoided by tapping the door when first interacting with it

Monsters

• You can run past Mary in hallways, as you survive 2 hits from her
• The background soundtrack will fade when she's present on the map
• You can also run past the Tortured Soul and the Dark Skeleton without dying
• Best way to deal with Skeletons (especially on Night 6 & 7) is to conserve your sprint beforehand, and quickly run past them, only looking back at them once you've passed them at a safe distance
• The Flying Skull is usually best dealt with by letting him get a hit off on you
• Note that the background soundtrack will fade when he's present on the map
• Fire Babies can be run past without getting hit, but you won't die even if you get hit by them once
• Asmodeus will usually be a run killer if you free him on Night 4 & 5
• On Night 6 & 7 he usually won't spawn in time to be a threat

Other tips I have found:

Candles: Candles despawn the Tortured Soul which isn't an issue per se. However, if you light a candle between two and three minutes or within seconds of finding the artifact on Nights 3-7, it will keep Mary from spawning until your candle burns out. Night 2 is more forgiving where Mary spawns a little after 3 minutes. Keep in mind Fire Babies move faster when a candle is lit.

Unintinded Luck: Obviously this game has a lot of luck, but there is unintended luck as well. Good luck happens on Nights 0, 4 and 5, as far as I can tell. The elevator will already be activated and only will require that you find the exit (unless doing 100%). I am still trying to find a way to get this more consistent. Bad luck happens when you fall through floor into the endless void. I believe this has to do with memory inputs from the game's side.

Asmodeus: On Night 6, he will spawn between 8 and 9 minutes. He will not spawn if you have found every important piece. On Night 7, he will spawn between 6 and 8 minutes. You can flicker your light to see where he is. Otherwise consider your run over.

Elevator Switch + Mary: For whatever reason, if you activate the elevator, it lowers the time Mary is active on the map while your character is breathing.

New Strat: Between Nights 5 and 6, exit to the main menu once the elevator floods the hotel and reenter the game. This saves a couple of seconds over waiting for the natural screen transition.

A few other tips I found, mostly regarding the monsters:

• As you may already know, the BGM (the ambient music in the background) stops playing when Mary (and in later levels Asmodeus and the Flying Skull) spawns. However, the BGM also always stops playing when you activate the Elevator Switch. I'm not sure why exactly did KMonkey implement this (I guess it may be to make the elevator activation sounds more clear to be heard), but it's quite frustrating because when you activate the Elevator Switch, you won't know whether Mary is around or not until she actually spawns and despawns, after which the BGM will start playing again.

This is probably related to what Conyo25 wrote above, that when you activate the Elevator Switch it lowers the time Mary is active in the Dungeon.

• If Mary doesn't know the player's exact location, once she spawns, she will instead head for the starting elevator. As such, it's very dangerous to spend too much time near the starting elevator, especially in the later levels where Mary's spawn rate is higher.
• It's also quite dangerous to spend too much time in a single room, especially a large one, because Mary always spawns in a room, never in a hallway, and she can spawn in the room the player is currently in if you're unlucky enough. This happened to me several times during my playthroughs of Dungeon Nightmares II - I spent too much time in a single room and Mary spawned literally right next to me.
• The same also goes for Asmodeus as well. In one of my playthroughs, in Night 6 or 7 (the Hell Nights), as soon as I entered a large room, Asmodeus spawned right next to me and instantly killed me.