Can you post your full roster? There's a few epics that may make for a better team. Ultimately I think you'll be building something around Salad though. 

I don't have many more outside of what I initially posted. Here they are...

i

Okay. Are you familiar with speed tuning? If so, I think you want to use this tune: https://deadwoodjedi.com/speed-tunes/single-43-advanced/

Specifically, you'll want to use this variant: https://deadwoodjedi.info/cb/55f338ec8f3b8d14a59e838a4cb319faf2481e4c

Sepulcher will be 4:3 between AoE, aka turn2 of each rotation, and will go last to put up block debuffs for two turns and block spddown and stun. You'll also want to drop tagoar and replace him with giscard (turn off his a2), and you'll want giscard last on turn1 of each rotation. Lastly, you'll want to drop frozen banshee for Ninja ideally.

How close are you to getting Mikage? Mikage (staying in base form) + Ninja is huge damage on CB, on top of NInja's already high damage. The only problem is it will lower Geo's damage since Ninja will likely overwrite his HP Burns so Geo might not be as viable, but the Weaken will still be nice.

The problem you're gonna face really is survivability. You don't have what I would call a keystone champion, the one that brings it all together. Team Block damage or Unkillable champs do this. There's also champs that do huge shields to extend your run or damage reducers with team Ally Protection.

Based on what I see, your best bet might be to try and maximize your damage in the shortest amount of Boss Turns possible. When the boss is killing multiple people with his AOE, reviving them isn't enough because they've lost all their buffs and since he does back-to-back aoes, they're gonna die again on the 2nd aoe (most likely) so it's not extending your run by much.

Ninja + Apothecary + Seeker (apoth/seek in max spd/def/hp and relentless if possible) plus 2 more champs, another solid DPS preferrably Atk based and then your Tank *might* work and get you 25 mil per key on UNM.

The idea being to do as much TM fill as possible a long with inc speed and attack so your 2 dps take enough turns to get your damage in before you wipe

I did read about speed tuning but it was too early for me back then. I'll look into that, thank you.

I'll start with what you have suggested as a starting point.

Yup, I still feel I'm missing something. I am far off from getting Mikage...

I am kinda doing what you suggested in the paragraph 3 by boosting stats whenever I can.

In case of Ninja + Apothecary + Seeker scenario, what other 2 are you recommending? I guess you are not focusing any survivability in this case?

Even if you go with a faster team you do still need to survive. Consider for example a team that can only last 10 turns. You would need to increase your entire team damage output by 10% just to make up for surviving to an 11th turn.

If you want to try that approach, Giscard is highly recommended since he brings def up, atk up, and atk down.

I couldn't access your second link... Something about not being able to login, even though I was logged in already.

Any alternative link please?

It's linked from the first one. You do need to login, but it's free.

Sorry I should clarify, you still need some survivability but revives probably won't do you much good. Korugar would be a decent way to go for your Tank. I'd put him in 2 Piece Retaliation and 4 Piece Regen. He's got a Turn Meter fill also plus soaks and reduces dmg with his Strengthen. 

How close are you to getting Mithrala? Honestly I'd go with Hotatsu. Decrease Atk and Inc Defense with a Heal would be solid. I know what you're going for with Sepulcher Sentinel but that's a lot of RNG you're hoping for that won't be consistent.

Of course make sure everyone has Warmaster for their Mastery. 

Korugar, Hotatsu, Seeker, Ninja and Apothecary. Korugar soaks the dmg and Apothecary full heals him, Hotatsu keeps Leech on the boss with Decrease Atk + Inc Def for more survival. With 3 TM Fillers that almost add up to 100% you should be able to cycle through pretty quickly to get more attacks off. Put Korugar in the lead.

Of course, you know your runs best. Only alternate I would say (especially since he's already built) is Tagoar instead of Apothecary. Tagoar shields people with his rez so that would give them a better chance at surviving the next hit. How much HP does your Tagoar have?

I'm very far away from completing Mithrala as well, 1/4 way through. I didn't focus on her early enough.

My Korugar is still on Lifesteal so that's worth a shot.

Everyone at 60 with single hit are on Warmaster.

"Korugar, Hotatsu, Seeker, Ninja and Apothecary " - that's an interesting combo. Worth looking further into it.

My Tagoar has just over 92k HP and speed at 174. I can enhance further, but currently saving them in case better ideas arise. :)

You can try Giscard for now instead of Hotatsu since he's already 60 on your account. They both do the Dec Atk and team Inc Def. Only problem with Giscard is his provoke is useless for the boss, he doesn't have leech or team heal and only does a self-shield. Hotatsu will really help keep everyone's HP up because when Warmaster procs on Leech, it's a huge heal for the champ

Another option is Jareg. The problem is his A2 is 4 turns booked BUT his Passive might put out lots of heals on everyone after each boss aoe (I'm not sure I've never tried it or looked up videos).

Hotatsu was a key member of my Shadowkin Faction Wars team so building him will benefit you there also (especially if you're doing well there with Ninja already)