So basically, you may have thought the Iris fight was the easiest in the game (even easier than Shura). But in truth, Iris is the hardest boss in the game.
You see, Shura is a boss you fight before Iris, so logically he must be weaker. This means Iris > Shura.
Lady Carina commits death upon her defeat, so logically, she must be weaker than Iris, who doesn’t die. Iris > Lady Carina.
Cernyx is so much weaker than Iris that he took hold of an island with no one living on it, while Iris melted an iceberg. Cernyx couldn’t do that, so Iris > Cernyx.
Argos is old. Iris is young. Iris > Argos.
Lady Carina & Argos are both bosses that are a higher level/at later progression than Lord Elius, yet Iris is proven to be harder than both. By that we can deduce that Iris is also stronger than Elius. Iris > Elius.
Now, you may be thinking, “sure, Iris may be stronger than those guys, but how is Iris stronger than Calvus?” Well, to that I say, Iris keeps all her clothes except a bracelet when she dies, while Calvus is striped away of everything. Clearly, Iris is more resistant.
Hopefully this helps you see the true hardest boss of Arcane Odyssey. Iris solos