Children of Sissna mode is an option from the main menu that is unlocked after defeating the third boss, Amiren and learning Shadow Walk from Silaren.
This game mode, which is played on a separate save file, is the same as standard mode except your health and stamina are greatly reduced compared to what you'd have normally. This makes things fairly difficult since enemies will kill you very quickly, particularly in the later stages of the game. Your AI companion is weaker than he was in standard mode, too, and dies a lot more as a result.
In my standard playthrough, I ended up having 392 health and 260 stamina. In CoS mode I ended with 141 health and 128 stamina- that's a little more than 1/3 total health and about 1/2 total stamina. I completed all journeys so outside of maybe some more health/stamina from missed Ashen Feathers, this is what you have to work with.
I played through most of the game on CoS mode myself and it was a slow going, tedious, and extremely frustrating experience. The final two bosses are an absolute nightmare and I'm not convinced that the 2nd form of the final boss is even possible to defeat by yourself (with an AI partner) in this mode. The saving grace is that you can engage in co-op with real players who are in standard mode and let them tank for you, but this only makes things somewhat easier.
Based on timestamps for some of the other winners, it appears that some people completed the game on CoS mode as their first completion, which was almost certainly done by co-oping with a friend on their standard playthrough. Again, this doesn't by any means make things easy, just less insanely difficult.
If someone does earn this achievement with strictly AI partners, I'll be impressed and shocked. Even with a standard mode partner you have to contribute enough that you constantly put yourself in situations where you can be killed in the blink of an eye, and because of the 'one revive, then dead' system, it's terribly unforgiving and borderline unfair.
I ended up using the Cleaving Axe for my entire playthrough. There were times when I actively farmed scoria to upgrade it and improve my damage output. I tried a one hand weapon with a shield and didn't like how it felt, but feel free to experiment and see what works best for you.
For talismans, I used Robust, Swift, Greater Vigor (swapped to Double Swig for final boss), and Steadfast. Greater Vigor only gave 8 additional health but every little bit helped. I used the Gefn's Blessing relic which builds up damage resistance over time and can mitigate any single attack once fully charged.
Because of armor stamina penalties, I used Defiler Veil over higher defense options. I swapped to Celestial Aegis for the final boss.
Remember that because your overall health is so low, White Sapiotes are a great healing option. Keep stocked up on them for when you need to heal less than a full gourd swig or if you run out of swigs altogether.
Since I ended the game with 141 health (149 with Greater Vigor) and 128 stamina, I didn't need to upgrade my gourd potency beyond 162.
I played through the game by myself up until the Seat of the Matriarch, where I had a friend on standard mode to co-op with. We made it through the dungeon without much issue but Amiren took us a few attempts. I then had to go it solo until the Palace of Lathyrus, which is where the difficulty really ramped up. After a few hours of unsuccessful solo attempts through the palace, I managed to hook up with a random player and we made it through to the Ritual Stone at the end. I immediately messaged him and we coordinated co-op for the Shadow of Ashen.
I tried to focus mainly on adds and let my partner try to keep the boss' aggro. Adds can get overwhelming at the end and I couldn't possibly go face with the boss without dying very quickly. This only took a handful of tries before we got it done.
Our strategy for the 2nd phase of the final boss was for me to keep a distance to avoid being one-shotted and throw Shield Maiden's spears to chip away while my friend tried to keep Sissna's focus. There's really no way to go toe to toe with her without getting decimated instantly. With him tanking, it wasn't that bad.
I expected this to be something like a lvl 1 playthrough of Dark Souls, which I've done multiple times with little issue. This was so much harder. You just die so quickly.
If you want to unlock this achievement, plan on finding a friend in standard mode to co-op with and even then, it's rough. Good luck.