What we're doing here is going for the learning branch of the immortality paths. This is just a step by step guide focusing solely on getting this achievement, but should give you a good enough idea of the process to get immortality with the other paths if you so choose. This guide does involve a bit of cheating the system to get around autosaves if you get unlucky, but still gives you a good understanding of the mechanics to get the achievement even if you don't want to game the system. And considering how much luck is involved in getting this achievement if you don't know what's going on with the numbers, I don't particularly feel bad about it.

1. Start off by selecting the Early Middle Ages start and the Lambakanna Kingdom all the way in the south-east corner of the map. Turn on Ironman then set your alternate rules to give you the best chance of not dying possible. Historical major epidemics, fewer minor epidemics, etc. MAKE SURE THAT YOU HAVE THE SUPERNATURAL EVENTS RULE TURNED ON!

2. There are two reasons for choosing the Lambakanna kingdom. The first is that Buddhism gives you a +4 bonus to learning and lets you select your primary heir. The second is that it is a small, out of the way country that you can build tall in and will mostly be ignored by the AI so you can just spam press through all of the events. You're going to be doing a lot of waiting for the event to fire so in the meantime you want to be working toward getting primogeniture so you don't have to deal with pesky factions and the like. Get that third point in legalism as soon as possible so you can switch to primogentiure. You also want to be setting up to get the best learning heir possible. With the first couple characters you are just going to be running through waiting for them to die so you aren't splitting your kingdom apart. Characters with high learning are easy to get. Once you have a six year old son, search for a guru then have him educate your child.

3. Once you have a high learning character, your goal is to live as long as possible. The older you are the more likely the event will fire. I recommend taking warfare to try and hopefully get the brawny trait. Once you hit fifty, switch to hunting to get a pet dog. But essentially it's a waiting game once you have a high learning character. Maybe the event will fire, maybe it won't. Just keep on raising children with high learning. Take concubines with quick or genius or strong. Anything to make your heir live longer or be smarter. Make your heir your physician. They get a health boost if they get the trait. Personally I always went with genius over strong because I wanted the learning boost. It is very useful as you'll soon see.

4. Huzzah! The event finally fires! Now what?
Well now you want to make sure you have the best Upadhyayani possible because they are the one finding you a mystic. As a buddhist you should have plenty of high learning characters in your court, especially if all your children have religious upbringings. So when the Meeting of the Minds pops up, select your religious advisor. You also want to switch to scholar and start building an observatory. You want to get your learning as high as possible.

5. A Holy Sigh
Pick the option that costs money. Higher success chance.

6. Now here is where you're going to start gaming the system (or not. Up to you. Guide still works without, you just have to have more patience). There are three options that will occur.
A) Your advisor will not find a mystic
B) Your advisor will find a fake mystic
C) Your advisor will find a real mystic
Note that the odds are always against you when it comes to a real mystic even if you have insane learning. The chance of successfully finding a real mystic are
13.46% at 12 or lower
17.88% at 13
23.37% at 16
29.92% at 19
37.41% at 22 or higher
If your advisor does not find a mystic, hard reboot your system. Don't click any options. Hit the power button on your computer. The way ironman works now is it saves on the 4th of January and July as well as a few days after loading a save file. Hard rebooting acts like a crash and you can go back to the latest save.
Alternatively opening task manager and ending the CK2game.exe under the processes tab. (Ending it under applications tab still causes it to auto-save). Further you can copy your saves from the Paradox folder typically located in my documents. Simply copy them to another place (Like your desktop), leave the originals in the save folder. Then you can go along the event chain. If you fail simply copy back the save files you copied before to the Paradox CK2 save folder, overwritting what is in there (Do this of course while the game is already closed). Then open the game back up and load the saves and you will be back to the autosave just before making any descisions with the event. (Credit to NoodleNaught this paragraph.) Any time it says hard reboot in the rest of the guide you can do this instead. It will probably save you time.
If your advisor finds a fake mystic, hard reboot your system. The way you tell if it's a fake mystic is by looking at their base learning stat. The real learning mystic gets a +15 bonus to their base learning stat (diplomacy, martial, and intrigue mystics only get +6 to their respective stats so it's harder to tell if they're real) which you can look at by mousing over their learning on their character screen.
Fake mystic

Real mystic

Just keep rebooting until you either get a real mystic or the autosave arrives. If the autosave arrives and you still don't have a real mystic just follow the chain through to your death so you can get to your next character. Note that once an event appears on your screen, the results for which button you click have already been decided. So if an event pops up and you save while the event is there and then savescum if you get an unfavourable result, the result will be the same every time you reload. However if your save is even just a day before the event fires, you can get different results from the different options.

7. You did it! A real mystic just for you! Just ignore your family member if they want to investigate. You know the truth. The next challenge is A Fallen Man.

Always pick the second option. It's a guaranteed success for the learning path only. It is not guaranteed for diplomacy, martial, or intrigue mystics.

8. Mind Over Body
Pick the dangerous option to keep fasting. It gives a higher success chance. If you fail, you guessed it. Hard reboot and try again until you succeed.

9. The Bonds of Mortality
I hope you're ready to become immortal because this is the final step. The chance of success on this is miniscule if the mystic is a fraud (as in nearly 100% guaranteed to fail), but if you have a real mystic and a high learning (22+ gives a 58% chance of success on the risky option) you should get it. So drink that draught and if you're lucky...
BOOM! Immortality achievement is yours. If you're not lucky, E.g. you are reborn as a child and do not become immortal as your current character, reboot and try again.

10 (optional). Becoming immortal also grants immortality to your mystic advisor, so it's a small step to imprisoning and executing her to then get the Eternity Denied achievement.




[Hopefully this guide helps you to getting the There Can Be Only One achievement. It took me two DLCs before I managed to figure out how to get it so it should be of help to you.]