This is the 4th pantheon in the Godmaster content and compared to the rest, it's pretty hard. Particularly the last boss, Soul Vessel.
You have to face 10 bosses back to back with a Hot Spring and Bench halfway through (you can modify charms at the bench but progress is not saved).
1. Enraged Guardian
2. Lost Kin
3. No Eyes
4. Traitor Lord
5. White Defender
Hot Spring and Bench
6. Failed Champion
8. Watcher Knight x6
9. Soul Tyrant
The Godseeker NPC
10. Pure Vessel
The only way to get through this is to practice and master each boss encounter, knowing their attack patterns and when it's safe to counterattack. In the main game it's often fine to flail through bosses and barely survive but to get through this pantheon, you need to defeat bosses with minimal damage taken. Pure Vessel will likely prove to be the most difficult part since he's, in my opinion, the most difficult boss faced thus far.
Pure Vessel always hits for two damage. His attacks are incredibly fast and there is little time for counterattack.
From the start, he has four attacks:
1) Lunging stab: Jump over and pogo him as he goes past, then try to attack once more after landing.
2) Triple slash: Again, jump over and pogo him, then try to attack once more after landing.
3) Dagger burst: Jump, then dash through the daggers and attack him.
4) Slam/pillars: Dash out of the way and find a safe spot. Attack if able.
Should he block two consecutive attacks he will unleash a sweeping counterattack, so if this is coming, hope to dash through it to avoid damage.
About halfway through he will start doing a focus attack where circles appear that deal damage after a second or so. Get out of range and try to see what he is doing next. I usually shot a Shade Soul or two at him.
About 2/3 of the way through it adds another attack where it lashes straight out with some sort of long void tendril. Double jump to stay airborne, ideally toward him to pogo on his head.
You can go to the Hall of Champions (below the four pantheon entrances) to practice any boss that you've faced before. So you need to get to Pure Vessel in an actual run to unlock him in the Hall. Then practice him until you know his patterns.
Per Awoo in the comments:
You CAN fight Pure Vessel without having to reach him in the patheon first.
When you clear the first three pantheons, you unlock Godseeker mode. This is a game mode (select New Game to enter it) where you start in Godhome with all charms and upgrades, and all bosses except Radience already unlocked in the Hall of Gods. You can just go there right away and practice the Vessel to your heart's content.
Arcane Insanity and Awoo give some other strategies in the comments as well.
Here's my successful run:
Here is a Pure Vessel battle on the Radiant setting:
Full info can be found on the Hollow Knight wiki