This is intended. The bosses should have required full plate armor with parry rate and so on. It makes me a little sad that some groups managed to kite bosses. But that's ok for the moment.
The AoE from bosses are intended the way they are. There are gameplay mechanics to lower the received damage though.
The boss is stacking dmg into cosmic values, I think there is a big bug in boosting the damage
The boss works as intended. It is supposed to be a direct damage check for the party and a tankiness check for the, well, tank. The boss gains 5% damage every second whilst losing defense.