ok so one thing that i noticed from watching others trying to do this challenge is that most people don't make use of creating soul links other than for leveling up/getting new abilities. in other words they don't think about saving the game because they think there is no point to save in one life difficulty, which is not entirely correct because most of the times in ori when you mess up and you know you are probably gonna die, YOU CAN JUST PAUSE THE GAME AND LOAD YOUR LAST SAVE. so lets say you miss a jump and you are falling into spikes, just pause the game before dying,then load your last save. or let's say you are taking too much damage in one area and you are not confident to continue with the amount of health you currently have, just exit the game and try again... you get the idea.
btw dont worry Im pretty sure this is not considered a manipulation/exploit whatsoever, otherwise the developers wouldn't let you to save the game in one life difficulty at all.
however there are some instances in the game that you dont have much time to react and reload the game. and these are the actual challenges of this achievement. for example the part when you are on your way to moon grotto (assuming you are taking the normal intended path of the game and not doing frog walk which is a speedrun strat) , when chasing gumo there is that laser part which you can die pretty quickly and you dont get the chance to reload if you don't get the timing right. especially in the late game in mount horu there are a lot of these instances that are insta death and you can't really do anything about them other than being decently familiar with the game and remembering the path you are gonna take to beat the game and always knowing what comes next. another thing that is very easy to forget and die to, even after beating the game several times is the lasers/flames in sorrow pass, they are insta death and you need to remember exactly where they are.
so its much better if you have already beaten the game on normal difficulty a few times and are somewhat familiar with the game otherwise it could get really frustrating on one life mode.
btw FOR PRACTICE ALWAYS PLAY ON NORMAL, because the one life difficulty is the same as normal in terms of the core difficulty of the game.
its not a bad idea to make a copy of a save before a section that is challenging for you and practice it a few times.

make sure to get as many life cells as you could during each run and forget about the ones that are too far from your path or are in bad spots and doesn't worth the risk, most of the life cells are in very convenient spots and are very easy to get, so make sure you don't miss them as it makes the challenge easier to some extent.

one final quick tip is i highly recommend right after you get the charge ability go straight to black root burrow and GET THE DASH as it makes the laser part on the way to moon grotto (and a lot of other parts later in game) much less scarier. its not mandatory for the run but it only takes about 2-3 minutes to get it, and it makes the rest of the run much faster and easier

even if you do the save/load trick that i talked about in this guide, this is still a hard achievement and requires a decent amount of time spent in the game and its 100% normal to keep dying again and again until you finally get it. but :
-knowing the path to beat the game (and not going to dangerous places that are unnecessary),
-collecting the easy life cells during each run (i would say 8 or 9 life cells is perfect dont bother collecting more, some of them are in risky spots,
-remembering the insta kill spots and making sure to not die like an idiot to them
-and most important of all make sure to save often and reload the game every time you know you are going to die (or low on health)
paying attention to these tips you should be able to get this achievement very soon, its definitely very obtainable.
me personally i love this game and i was obsessed to get all the achievements (and i did), now i just do speedruns and the one life difficulty is not even a challenge for me. if you had any questions i'd be happy to help you ;)