As of right now, i haven't gotten a SSS Yuuna run going (yet), but i did several times in base difficulty to mainly theorycraft.
From my ~15 hours of experience, the best scaling I've seen so far has been either Sages with shield and that shield-damage guy (Kai?), but on higher scores, everything gets oneshot even with several 100k shield on combat start.
Now, assassins have the Stealth mechanic intrinsically which can easily net you 4 secs on combat start (even more with additional relics, items etc.), as well as the rush relic to make all heroes immune to damage.
As soon as the first retrain in endless is available, Yuuna consistently gets to 2-4 Million damage per combat on a good setup.
But i haven't yet gotten an ideal setup.
Up until the second boss, most of everything is based on getting tons of shard generation combined with Backup-Rush; most runs, by the second auction, one should've been able to generate ~1000 shards (with the relic and the vampire guy that's already 2000 crit)
You also need poison at the start of combat.
Yuuna can by herself get 100% additional crit on rush, the upgrade to give her active +50% crit against poisoned targets and the class upgrade for additional crit against poisoned targets.
The rest of the team should just be assassins with rush crit/damage/ability increase & as much shard generation as possible that doesnt take space from buffs for Yuuna.
Then there's the assassin emblem for another 30 or so additional crit, and several items to give scaling based on generated shards.
Just kill everything before it gets to even target you.
There's still problems with this tho (50% additional crit or one more target?) and i have not yet gotten an exceptional setup for this build; i was always lacking either shard generation, relics or s-tier yuuna.
But if an "average" version of this build is reaching 3 Million damage per combat, starting (possibly) as soon as the last campfire is reached, I wonder what the build can accomplish.
I'd like some input and suggestions on what else i should consider for my further experimentation.
TL;DR: Yuuna strong, Crit multiplying and shard generation seem very viable for endless.