• First of all, it should be obvious that you need to know the game very well before even thinking of attempting this; other achievements require you to go through the game a number of times if you want all of them, but that's still not enough: you need to know the game by heart. And you need to master Ori's movement and abilities to have the confidence necessary for every situation. So in short, you need to play this game a lot.
  
  
• Prepare a savefile for each section that you might find especially difficult. This will allow you to practice it every time you want, and particularly when you have to go through those specific sections on your one-life run, for maximum confidence. For instance, I personally kept saves just before the first 2 lasers on the way to Moon Grotto, the Ginso Tree escape, the Forlorn Ruins escape, the section just before Mount Horu, Mount Horu itself, 2 specific rooms in Mount Horu and the final escape sequence.
  
  
• Remember you can pause the game and exit at any time, so if you're not feeling confident at one point, just save and try to go; if you see you're ending up in a tough spot or your health is too low or you're falling into lava or other scary stuff, just pause, go back to the main menu and try again.
  
  
• Watch speedruns. Since you'll probably die a lot and end up repeating the same things
  over and over again, learning how to do it quickly could prevent potential loss of sanity. If you're already a speedrunner, you're awesome.
  
  
• ALT+U hides the UI - this will automatically skip all of Sein's messages and various textboxes, so you don't have to break your spacebar to quickly advance the dialogue each time.
  
  
• Delete the savefile each time you die. This is actually really important for 2 main reasons: first, it will prevent a bug that causes your savefile's difficulty to change when you die, it will still say you're in one-life if you check, but you won't get the achievement at the end. Of course this really doesn't matter if you just want to do a no-death run and don't care about the Steam achievement popping up.
  Second, whenever you die in one-life mode the game will automatically put you back at the beginning, and leave a "Wisp" in the exact place where you died... think of it as the equivalent of bloodstains in Dark Souls... it contains some experience ("spirit light") probably based on how much time you managed to stay alive the previous time, or how much experience you had, I don't know, the important thing is that they are evil. Thanks to the fact that they spawn in the exact place where your HP reached zero, you will find them in the most dangerous of places, specifically on top of insta-kill lasers. This wouldn't be a problem if you could just walk over them, but if you come in contact with them Ori will stop and will execute a pickup animation. You don't want this to happen in front of a laser. You won't have enough time to jump out of the way and you will die again in the same spot. This is not a problem if you're sure you died in a safe spot, but to avoid wasting your time for nothing it is generally better to just delete your save and start a new one.
  
  
• Grab as much spirit light containers and ability cells as you can along the way, but no need to risk your life trying to reach the really tough ones.
  
  
• The order in which you get your abilities doesn't really matter, but I usually aim for:
  - Ultra Spirit Magnet first, just so I don't have to worry about losing experience because I'm going too fast; this way you'll also have Life Efficiency and I don't need to explain why that's really helpful.
  - Then second is Ultra Soul Link, which will allow you to heal 2 life cells whenever you want, provided you have energy; this will also give you Charge Flame Efficiency allowing you to use Charge Flame more freely.
  - And finally I dump everything else into the attack abilities.
  The reason for this is that you don't really need to use the Spirit Flame that much, most of the time you can dispose of enemies quickly with Charge Flame, Stomp or Bash. Nearing the end of the game having a powerful Spirit Flame can potentially save your life though, so that's why I leave the attack upgrades for last. I also decided to not go for Ultra Defense since the real danger in the endgame are not enemies but insta-kill environmental hazards, rendering that ability pretty much useless. I also didn't feel the need for Triple Jump at all.
  
  
• Be careful while bashing off those green exploding enemies, if you aim too close to the ground they can explode in your face, and they will oneshot you if you don't have enough health. Always aim a little bit up when launching them.

Area specific tips follow below.

• The first thing you might find useful if you hadn't noticed is that you can reach this Energy Cell here from below after you get Double Jump, instead of risking taking damage from the spikes coming from the other side on the right. This is probably something really obvious but it took me a while to figure it out... so I'll just leave it here. Just double jump from the crystal to the wall directly above it and then you can climb up no problem.
  
  
  
  
  
  
  
  
  
  
  
  
• Another thing that might be obvious but it's a little trickier than the previous one, is reaching this Life Cell (thick dot means double jump):
  
  
  
  
  
  
  
  
  
  
  
• After you get the Water Vein, drop down here and all the way to the left and you'll find yourself in front of a free Ability Cell:
  
  
  
  
  
  
  
  
  
  
  
  
• On your way out of Moon Grotto you'll find a locked door that needs 3 energy to open, behind it lies a Life Cell; you want that Life Cell.
  Normally you'd have to make your way down and then back up from the left side of the room, double jumping over poisonous water and climbing between spikes and shooting blobs, potentially risking your life every jump... but you can actually reach it directly from the right side and skip all that nonsense with a few very precise double jumps.
  I've marked the spots from where you need to jump and where you need to land to make it, I wouldn't say it's a pixel perfect stunt but you need to be precise, it could take a few tries, but you're not risking anything. Again the dots mean double jump
  

• There's an Ability Cell you can reach right after getting Double Jump here:
  
  
  
  
  
  
  
  
  
  
  getting the second double jump right is very iffy though, so if you see you just can't make it, forget it and move on.

Nothing much to say here, just be sure you know the escape sequence by heart.
When you get to this point though:











don't rely on the spider to shoot another ball upwards so you can bash off it; instead get on the right wall, climb up a bit and just double jump towards the glowing branch, you can reach it without the spider's help.

• Be careful around this spider:
  
  
  
  
  
  
  
  
  
  
  
  Usually you should be able to just bash off him upwards and gain enough height to reach the glowing branch above, without the need to bash off his shots too; I'm saying this because sometimes he doesn't want to cooperate, if you're waiting for him to shoot up.
  
  
• Be careful when bashing off the 4 birds immediately after; if you miss one, and you're left falling down, Kuro will oneshot you before you can even realize what's going on.

• When you reach this point:
  
  
  
  
  
  
  
  
  
  
  
  you would normally have to go down and right and navigate through moving platforms and spikes... that's dangerous. There's a faster and safer way up, using that jumping enemy: just wait on the edge of the wooden platform for him to jump on you, then bash off him diagonally up/right, and try to land over the spikes. At this point you can climb up in the secret passage here:
  
  
  
  
  
  
  
  
  
  
  
  and proceed normally. Be wary of that exploding enemy.
  
  
• Also, there should be no need to tell you this, but... be very careful when carrying the torch. This is one instance where having a powerful Spirit Flame can make your life easier, but dropping the torch for a moment to stomp the blobs to death is also an option I guess.

• This spider wants to ruin your day:
  
  
  
  
  
  
  
  
  
  
  
  Obviously don't try to cross this while the spider is alive and you're carrying the freeze-ball-thing. Instead bash off the spider to that safe spot and throw his shots back at him until he is no more.
  
  
• There are a lot of one-hit-kill lasers around here. I assume you already know how to get through the level, so, just... be careful around those things.
  
  
• There is a Life Cell to the right of this room:
  
  
  
  
  
  
  
  
  
  
  
  But you see the lazors, right? There are more on the way to the Life Cell. We're not messing around with that. Forget about it and proceed towards the end of the level.

They don't call it Sorrow Pass for nothing.
Spikes here deal 4 hp of damage, and they're everywhere.
There are surprise instadeath lasers.
And the green frogs are mean.

• First things first, go up and collect the 4 keys available to you right away, but DON'T use them to open the door you would normally open: we're gonna need them to skip an annoying part later (you know what it is).
  Instead go down here (be careful of the 2 intermittent lasers on the way):
  
  
  
  
  
  
  
  
  
  
  
  Don't kill the frog; use its attacks to break the floor above you first, then proceed normally.
  
  
• And now we come to the part you all have been waiting for:
  
  
  
  
  
  
  
  
  
  
  
  This is what we have been saving those 4 keys for... normally you would have to carefully float all the way around this spike hell to gather 4 more keys to open the door... but we already have them now. Just gotta be careful while reaching the door.
  
  Alternatively, there is a way to skip this section altogether, with a method I've seen used in speedruns, which involves dragging a green frog all the way up here with bash (it has just enough health to survive) and use its shots to break open the upper floor. If you can do that, you're awesome.
  
  
• Be wary of the last 2 intermittent lasers on your way up, and then down again.

This is where, as they say, "♥♥♥♥ gets real", because instant death is everywhere.

Let's go room by room:

• First Room
  Not terrible, just don't mess up your jump timing, but I feel like I should point out that after reaching the lever on the left, you're supposed to go across as the platform comes towards you, and then bash upwards using the lantern on the right; I say this because I've been doing this room in a more complicated and potentially way more dangerous way for a good while, so in case anyone thought of some other creative way to do it: don't.
  (thank you to everyone that pointed this out in the comments 👍)
  
  
• Second Room
  This one's not hard, just keep an eye on the laser.
  
  
• Third Room
  This is probably the worst one... the floor is lava pretty much everywhere and there's enemies shooting at you from everywhere. Bash is what keeps you alive here; in case you have troubles, here's a video:
  
  
  
  
  
  
  
  
  
  
  
  
  
  
• Fourth Room
  The little escape sequence shouldn't be too dangerous, but the rotating lasers after that sure are:
  
  
  
  
  
  
  
  
  
  

• First Room
  Those spikes dropping down are annoying. They're instant kill of course, and you can't even touch them on the side, so be careful not to slip down the side of their platform on your way back up.
  
  
• Second Room
  Another rather easy room, for a change...
  
  
• Third Room
  Be quick and precise in the laser section...
  
  
• Fourth Room
  Those red lasers will "only" deal 4 HP of damage, so you can afford to get hit once or even twice if you have enough health. Be careful bashing near the next laser though...

Nothing to say here, you should just learn it by heart.

You can do it ヽ(ʘ ͜ʖʘ)ﾉ