So I just cleared Juleeve Council, one of the new raid bosses, and thought I'd write up a mini-guide about it. He (it? them?) can be found in the bottom right of the map. Being one of the harder bosses, he's lvl 54.
The fight is basically comprised of 4 mechanics:
1. lots and lots of poison. The boss' attacks poison you and it spreads among team members, meaning you'll have to find a way to deal with it and the sooner the better;
2. the boss is immune to CC when whole, and debuffing him will cleanse it off AND apply a buff on the boss, so avoid bringing units that debuff on S1;
3. a three way split at 40% and 70% hp tresholds;
4. a stacking spd and atk buff, which seems to reset at each split and is manageable, but it makes the fight somewhat of a dps race if you're not equipped to deal with the poison
To counter the poison I brought both Destina and Diene. The idea is to use Diene's single debuff cleanse when the boss is in his single phase, because poison application is manageable, but save Destina's full cleanse right after the end of the split phase so Diene can take over again. An immunity buffer could work aswell for the single phase, but the split phase gets hairy because: a. your party loses one buff each time you take a turn, making the immunity buffer useless for this phase and b. the boss splits into 3 parts and every time one of the parts attacks it inflicts poison. The good news about the split is that the boss loses CC immunity, so you can control the fight a bit. This phase lasts until the "main" part is killed, at which point the boss becomes whole again. I'm not sure if this is random or not, but I killed the middle part first and it ended the phase each time.
The drop was a T5 unique epic weapon (atk set) with pretty much maxed but random initial rolls.