Master of the Path

I have linked below how to do the 10 fights and also top link is to easy cheese strat for all 10 fights courtesy of Quinn

It has been confirmed and tested the new brazier does work for this achievement. It is the one on the right as you face the three brazier of progressive power.

Mad Mortimer - New way with Pelagos
https://www.youtube.com/watch?v=TfFiju2Vjpk

Echthra updated fight on Humility with Pelagos and master of the path
https://www.youtube.com/watch?v=-SLgm_BStCc

Thran'tiok - This is just an easy fight
https://www.youtube.com/watch?v=BLGuuBdcahY

Kalisthene - Did this with Kleia
https://www.youtube.com/watch?v=iFiZgLyD-Hg

Alderyn and Myn'ir - Just go ham use aspirant's binding to hold myn'ir in place for stacking unleash
https://www.youtube.com/watch?v=4bGabeD9e90

Nuuminuuru - Aim to kill groups of Faeries
https://www.youtube.com/watch?v=YJ5DHmi_c5k

Craven Corinth - Go ring of warding and do shield, haste, haste, and shield as he does his 2nd fan of desolation. Use unleash when he summons his anima orb
https://www.youtube.com/watch?v=IyR4XoMaDVI

SplinterBark Nightmare 9.2.5 - Use Kleia and just get the full stack of flowers while dpsing him
https://www.youtube.com/watch?v=srR5zu2h8IE

Azaruux - I used Kleia and used his adds to get a ton of crit
https://www.youtube.com/watch?v=pMcEyvbAQPQ

Athanos - I used Kleia and moved him near a wall to keep dpsing him at all times
https://www.youtube.com/watch?v=7-IZ5Df_87Y

Rejoice- this is now trivially easy at Level 70! I did this fully today at 351 ilevel (idk if ilevel matters or not.) At level 70 you have high survivability, rarely taking damage, and you have enough dps to easily finish each fight in time.

Dragonflight and Beyond Guide
Ignore all posts older than this one, they will only confuse you.

1) You must unlock Humility level by completing quests / upgrading your sanctum etc. Check Wowhead Path of Ascenion guide for Details. Then, you must defeat each boss at highest difficulty. See below for details.
2) Choose Kleia for every fight, she has the highest dps.
3) There are three braziers that can be clicked off. You must click off two of them. The one called "Brazier of Progressive Power", you should leave lit. This is intended. There are also braziers for decoration that cannot be clicked off. I expect if you are coming to this event post-shadowlands, it is very confusing.
4) Choose appropriate boss at highest difficulty with the mirror thing.
5) You do not need to use any equipment. It's that easy.
6) Start fight
7) Go up to boss and spam ability 1. That's it.

A note on Athanos
He has an attack that can push you from the platform. You can dodge it, or, put your back to a pillar.

A note on Thrantiak
Kill the phylacteries before you kill him.

A note on a strange bug
Because we have more dps than expected, I noticed a couple of times that the NPC told me I lost, when I had really won. The credit on the achievement still updated correctly. HOWEVER, the system thought I had lost, in terms of unlocks. This is easiest to reproduce on Athanos. If you kill him right as he sweeps you off the platform, you can "win" but lose. TLDR; if a boss is not unlocked for you that you think should be, check all easier bosses to ensure the game really thinks you beat them.

Have fun!

It might be worth noting that the first brazier from the right can be turned on at all times as of writing this post - this is the "new" brazier added, probably they never coded change from 2 to 3 braziers.