Played this game for about 2 hours yesterday (great use of my time I know) and I figured out an extensive guide for Night 6 (and probably other nights as well).
Some general notes:
Night 6, wear headphones, you can hear what direction Bonnie and Chica come from when they approach the door.
You WILL also want to start a stopwatch right when the night begins.
Each night lasts about 8 minutes and 35 seconds (timed it myself) and each hour is about 1 minute and 26 seconds (also timed that myself).
If you refuse to check the cameras to save on power, you are most likely going to lose, Foxy is way too aggressive on Night 6 and will steal more and more power every time she hits your door. leaving you with 15% at 4 AM and no chance of winning.
You actually have 1000 power, and your current Usage amount is subtracted from that amount every second (in the original FNAF 1 at least), so having 2 bars of Usage will literally double how quickly you run out of power compared to only having 1.
Tips for Foxy:
Here's my strategy for Foxy: Count to 6 seconds in the office, then open cams and check Booty Cove for 3 seconds. Repeat this for the ENTIRE NIGHT. (I actually got this idea from a MUCH older comment in this thread, but it worked for me so ill post it again)
Foxy can be stalled for a long time using this strategy, but it also relies on some luck so if Foxy attacks before 2 AM, you should probably just restart (click the donut 5 times for an instant game over)
When Foxy eventually leaves Booty Cove, close your camera and hover your mouse over the Left Door until either:
A: you hear 3 quick footsteps, where you should close the door, wait until you hear all 3 bangs, wait an extra second, then open the door, or
B: you hear either Bonnie or Chica approaching the door, in which case you should close the Left Door immediately, then (if Chica is there) close the Right Door, then go back to the Left Door. If Bonnie is NOT there, open it, and go back to waiting for Foxy's running sound to play.
Foxy Note: Foxy will deal a scaling amount of damage to your power, dealing 3% total the first time she hits your door, 6% the second time, 9% the third time, etc. For this reason, you want to minimize the amount of times Foxy leaves her cove.
You should be using about 15% of your power every hour, leaving you with 2 Foxy bonks worth 9%, 6 Hours of 15% power usage worth 90%, 1% used on keeping off the animatronics, and anywhere from around 10 seconds to 30 seconds before Freddy kills you after the power runs out.
(It's best for Foxy to attack 3 times per night, once during 2 AM, again sometime between 3 and 5 AM (preferably closer to 5 AM), and lastly late into 5 AM when you have a low amount of power. If Foxy hits your door and steals 9% of your power when you only have 2% left, it leaves a significantly smaller impact than when she does the same at 12% left, so try to time Foxy's third attack when you are REALLY low on power.)
Tips for Bonnie and Chica:
The two doorway animatronics can attack and leave VERY quickly, so check the lights of whatever door you closed every 2 seconds (while doing your 6 second count for Foxy).
If you hear an animatronic coming while you have the cameras up, quickly close the door (do NOT check the light because if they decide to be extremely aggressive they can just jam the door and kill you, just listen to where the sound came from and close the respective door) and reset your 6-Count again.
If Bonnie is at the door, the white graffiti in the bottom right corner of the window will disappear (and you can actually see Bonnie's shadow but the graffiti is MUCH easier to see quickly). Wait for the graffiti to appear in the window again to know if she's gone. Bonnie LOVES to wait at your door for long periods of time and waste your power, so you'll need some luck if you want to survive as well (hence why it took me 2 hours)
If Chica is at the door, Chica will be visibly standing in the window (obviously), but she tends to leave much faster than Bonnie so check the right light more often when she is at the door, she'll probably have left within a few seconds.
And finally, Tips for Freddy:
Freddy is essentially a non-threat in this night if you've played it enough times. She always moves at the exact same time on every single attempt of Night 6, and she always follows the same path:
Stage, Men's Bathroom, Dining Room, Backstage, Closet, Kitchen (eating some Special Sauce), Attempting to attack your Office and (if the right door is closed), back to the Kitchen, where she will not move again before 6 AM hits.
Remember when I said to start a Stopwatch at the start of the night? Freddy will ALWAYS attack when your stopwatch hits 7:35-7:45, during around the middle of 5 AM, so close your right door when your stopwatch reaches this area (or until you hear Freddy laugh).
Once you hear Freddy laugh, that means she has failed her attack attempt and went back to the Closet, and you can safely open your door (unless Chica is there, in that case leave it closed), Freddy will not attack you again for the rest of the night.
If you've forgotten to start your stopwatch at the very start of the night, you can still save yourself from Freddy. When it reaches 4 AM, start a stopwatch right after you hear her laugh. Once your stopwatch reaches about 1:10, close the right door and open it once you hear Freddy laugh, again Freddy won't attack for the rest of the night after this.
Tip for Golden Freddy(?)
If you are unfortunate enough to have Golden Freddy show up during Night 6 and you DON'T want to see her jumpscare, open up the camera, switch to Cam 3 and click on the Golden Freddy poster and she'll leave. (You can always just set 6969 on Custom Night to see her jumpscare anyways, but you have to beat Night 6 first to do that)
Extra for Foxy:
After Foxy attacks a third time or it reaches 5 AM, I stopped checking on Foxy and just hovered my mouse over the Left Door for the rest of the night, waiting to close the door if she showed up and just hoping that she didn't.
I ended up getting to 6 AM right after Freddy's song ended (after my power ran out) with just a couple of seconds to spare using this strategy.
Hope this essay helps someone out.