Yeah, it seems I'm just not strong enough to do those high-difficulty missions.
Counter, evade + counter, attack drop, solar drive, total power - none of those strategies make any major change to the outcome, which in the best case was about 2/5 of the last boss's HP in the end.
Theoretically I still could try level breaking and ancient amping, but I don't think a maxed-out Saffron would make much of a difference when dodges really don't happen in this fight and the boss is able to one shot the rest of the team in the first retaliation.
I only hope these missions will come back at one time so as not to lose those ancient ampies.

Too bad this happened before the 4th anniversary events, since the target of my FG stacking - the discounted gacha - has a chance to appear.