So the Gyan variant looking jerk happens to be Barandos the Red, accompanied by Barandos' Dog.
I still lose my first attempt. Kind of. I give up early, but I knew how to win easily after that.
The dog just attacks for not much damage. It's a distraction. Barandos, though, spams Nereids of Thunder endlessly. Sarah is the only one hit particularly hard by this at the moment, it does half her HP usually And he usually goes first. Using Nymph Para I was able to drain enough HP from both to keep her alive. It was also draining his MP...The problem was when he ran out of MP, he started attacking. Barandos double hits. Everyone he double hits he does -at least- 100 HP PER HIT. That's enough to outright O.H-K.O Luci and Sarah, and even Coryool cannot stand up to it for very long. In the end the strategy was have Sarah cast Defense Up on herself, which boosts both Defense and Mind (and yes, Mind is both Magic Attack and Defense, so it technically makes this the overall best stat to buff/debuff in most cases), so she takes less damage and then boost Randoms Attack and Speed with Attack Up and just go ham on Barandos. It still takes a long time, because I think we're doing this battle much earlier than intended, but...
the end