These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Conquest. He attacks twice per round, can change his element frequently, even twice in the same turn, and can seal any of your battle options.
Famine. She's one of the few bosses who attack your mana, and can actively change your own element to Aqua.
Death. He carries both an HP to One attack and a powerful attack with a good chance of instantly inflicting, well, death. This is compounded by the fact that your first battle with him is one of the few times you don't control Darwin, who is your most versatile character.
To make things even tougher, Famine and Death are fought as a Dual Boss in Abbadon.