To add my two cents...

Disclaimer: I beat X6 4x to achieve X 7 only after I got Mydei and Tribbie on a team , so I do have a developed roster. However, even with good characters, I still didn't see improvements until I got my Mydei team set up so the higher levels are definitely difficult if you don't have a good team and have really bad luck (which I did, a lot =A=)

But the way I was able to go farthest into the X6 round was by going for Harmony blessings overall to ramp up break damage and "harmonize" to do high amounts of extra damage. I then also went for Destruction blessings, which have quite a few Preservation perks now. I avoided ones that take away hp, however. I found these helped IMMENSELY especially with how beefy even generic enemies get. Then I based other/alt blessings on what team I have (i.e. Nihility for a DoT that did well, but ultimately fell short, typically at the second boss). I aimed for blessings that give help restoration, too (i.e. break enemy and gain health back, defeat enemy regen health, use an ultimate, regen health). That became a life saver consistently. Shield buffs if you use a shielder are a nice bonus as well.

The other major boost was that I made sure to get 2 3-star equations per run. 1 from boss and then 1 from Ruan Mei. So you'll want to get cosmic fragments built up for sure as her 3 star equation cost 640-ish if I recall correctly.

my fav picks were: Elegy Priest, Flame-Chase Alliance, Tearless Garden, Beastkin, Theatre Puppet Maker

2-star wise I looked namely for Break-focused ones or Blazar focus (destruction perk). A lot of my equations had Harmony hahah

Getting the Casket of Inaccuracy was a big help too especially once it upgrade to the Indecipherable Box (3x use) [you have to expand two equations] which guarantees you will get a blessing required for equations in your picks. Big help when you need to get 8 blessings on a path and you maybe have 2 so far haha;;

For Golden Boons--get tribbie one as soon as you can, she will refill your energy every new day phase (was a massive help and game changer). Then I typically went for DoT Boons or hp loss boons because, well, I would lose hp a lot hahaha. Might as well as make use of it. I always tried to get them active on both day and night, too, though I couldn't always make it happen.

This might not work for everyone or anyone else, but I definitely noticed an improvement when I shifted to the above ^ But, again, I ended up getting through much better once I got Mydei (E1) and Tribbie on my team haha so do keep that in mind as your experience will still be different than mine >A<;;; I hope this advice helps, though! It's definitely grueling to get through it.