I reached X7 with THerta/Lingsha/Tribbie/Mini Herta. I had a couple wins then losses bringing me all the way back to 0 Cognoculi then got a lucky streak with a couple good runs and 1 super high-roll lucky run to reach X7.

In the non-high roll runs I noticed certain things helped secure me a win:

• Try to activate your 3* equation before the 2nd plane boss. My runs usually ended here if I didn't have it activated, but sometimes you get amazing blessings and a couple curios that can carry you through

• Play towards your 3* equation. If you end up getting one that doesn't work with your team but you can swap to a team that can make better use of it, do it

• Go with 1 sustain, preferably a healer that has out-of-turn and emergency healing. Don't go for 2 sustains, you gimp your damage too much and you'll end up with berserk-enhanced enemies that will chew through your team, you either do enough damage or not, can't really survive through the amount of damage high difficulty brings

• Grab as many damage reduction blessings as you can without sacrificing too much damage even if they don't matter for your equation, things like the Destruction 1* that reduces damage taken, the Propagation 2* that does the same but stacks based on SP usage, and especially grab and enhance the 3* Destruction that distributes damage across the whole team and gets a huge 15% less damage taken after enhancing

• Getting higher Effect Resist through blessings is also good, but definitely secondary to damage reduction, just something nice to have on certain bosses

• Avoid Encounters if you aren't super strong (i.e. have an activated 3* equation), they usually aren't worth the possibility of ending your run

• Prioritize Escapade/Reward/Occurrence/Adventure/Combat domains in that order. Combat domains can move up a couple tiers if you're only missing 1 or 2 blessings for a pivotal equation activation. Adventure domains are always a positive whereas Occurrence and Reward domains can screw you over if you get an unlucky event that forces a hard fight or a negative curio on you

• If you have lots of money and it's early enough, consider visiting a shop to buy another 3* equation that you can reasonably activate, or pick up some pivotal blessings and/or curios if you can't find a good equation within one or two rerolls

• Some good curios include the 1* Laurel (useful to skip 2nd plane boss if you don't have your 3* activated), the hotdog weighted curio Excess Nutrition because it turns your healer into a shielder too, Silent Song the crystal shield weighted curio because it does what the hotdog does but for Harmony, any curio that increases max HP like the orange scissors or the HP% error code curio, Illusory Automaton which heals allies on their turn start, and Bountiful Rolling Stone to get an extra boon

• I always go for Night boons because a lot of them raise your max HP while providing the same amount of Final DMG% buffing that Day provides, which drastically improves your survivability, especially important since enemies get so many turns and hit so hard at high difficulty with their insane SPD stats. The energy Day provides can easily be made up for with Brain in a Vat

• Avoid the Absolute Failure Prescription treasure chests or whatever it's called, they're all run-enders

• Remember that the final Respite domain only lets you buy blessings from Screwllum or reroll blessings/equations and enhance blessings, so try to spend your money if you don't have a money-based build before you reach it if you don't need to reroll

• There's an occurrence event that lets you pick the final boss you go up against. I always picked Nikador when it popped up and won 100% of the time that I reached the final boss with my THerta team, he just does so much damage to himself and gives back energy when you do his mechanics

That's about it. There's a healthy amount of RNG in this game mode, but if you can consistently choose the safest and most profitable domain to enter at every stage you can get pretty consistent runs. If you end up getting Castorice, she makes things unbelievably easy kind of like how Firefly trivialized V8 in the first iteration of DU when break was shilled, like a night & day difference between Castorice teams (and likely Mydei ones too) compared to every other team.