the first boss was a wall where actually you was forced to leave the whole dps on the boss to the tank only!
The way I always dealt with this was to tell the DPS in my parties to never DPS boss while adds were out; in addition, as tank, I'd turn my back to the boss until adds were dead. Worked like a charm.

But yeah, the Siren boss at the end had some more interesting mechanics than the vast majority of dungeons nowadays. The last two bosses in the new Ampador dungeon are just... laughable. The final boss' big move is to... cure himself a little bit. ???? What's the point even?