UPDATE: Apparently, since the game's update, This achievement, along with Speed Demon are given to you for simply beating the game. I no longer have the game, so I can't confirm this. Regardless, here are the guidelines I used to get the achievement before the update.
These are guidelines, not steps, so please read them all at once.
0a. First and foremost, do not attempt this achievement on your first playthrough of the game. It's not impossible, but it will make your experience miserable. Enjoy your first play, it's a fun game.
0b. Secondly, going for the Light Seeds Master achievement in your first play is incredibly good practice for this achievement. By obtaining all of the light seeds in the game, you will be quite well-versed in the layout of each level.
Once you're ready for your Gentle run:
1. Make this a dedicated run. Don't try to get other achievements. If you do, sweet! But don't let yourself get killed 10 times while trying to dodge The Warrior's attacks.
2. Grab a sheet of paper and keep a tally of your deaths. In all reality, you shouldn't be dying too much, but it's best to know where you stand. However, if your significant other sees it, you might have some explaining to do, and explaining notes for video games is never easy, trust me. So be careful with that.
3. Save often. The game remembers your exact location whenever you save, provided you're not in the midst of a boss battle. Do not use the autosave feature at all. Make it your own responsibility to track your progress.
4. Save any time you're ready to enter a new corrupted region. If you've been practicing (ie, got the light seeds from your first play), then you have no excuse to fall victim to your environment. You should be able to get from entrance to fertile ground without dying. On top of that, any run from the entrance of a region to its fertile grounds should never take you any more than three minutes. Should you die, reloading will not waste too much of your time.
5. Save immediately before you fight a boss. You'll know that you're approaching a boss fight when you can see a plain-as-day path to the fertile ground (a blue pillar of light) in any corrupted region. Boss battles can go horribly awry, so it's good to have a quick retry plan.
6. Save immediately after any boss battle. There's no need to fight a boss again because you forgot to hit the Y button for that large gap while you're leaving the region.
7. Be unforgiving toward the beginning of the game. You really shouldn't die at all for the first four regions. You can take any enemy down with the YA-YYY combo, all bosses but The Warrior included. You're technically still in tutorial mode here.
8. Don't get too hard on yourself later on. Toward the end of the game, the random corrupted that spawn can give you some heartache, the bosses take longer to go down, and the green/yellow plates can be downright frustrating. Best practice here is to allow yourself a certain number of deaths for each region. Even though the game gives you infinite lives, force the old school gaming mentality upon yourself: you die 3 or 4 times, then you lose. Reload.
9. Just to be safe, count the first time Elika saves you (crossing the bridge into the desert) and the time she saves you in the final Concubine fight, as well.
All in all, it's a tricky achievement, because it puts you in charge of monitoring how often you die.
If you can keep track of this statistic, and if you save often, then there's really no reason you can't do this. I had about 35 deaths on my run, and I stopped reloading for the last 3-4 regions and the final boss.
Achievement will pop once you've brought the final light seed to Elika.