I interpret the difficulties differently. I take the highest difficulty available for a boss and consider that to be their full power, the lower difficulties are just that boss but weakened by whatever ratio they decided to use. So easy Arkarium is easier than normal Arkarium, but may still be harder than normal Hilla, with the conclusion being Arkarium > Hilla.
But that doesn't change the fact that Arkarium's near-full map OHKO attack is beyond ridiculous. Just artificial difficulty at its finest.
The interesting fact is that Arkarium's "Full Map Attack" isn't actually full.
Emboldened the technicality. But then again, maybe you were just making sure everyone else knew. w/e
If I redid him then I would quite simply trash that trashy skill and implement a much more telegraphed-random OHKO attack. Or maybe give him something more unique to his skills. Either way it would probably end up being a skill that is pillar-style, because those tend to be the easiest to telegraph to the player.