Just solo Guard Golem Ultimate recently and i'd consider it the hardest boss i have ever fought after almost 8 years of Toram. Even with the easiest solo class(Knuckles), it was still hard. One smallest mistake is enough to get stunned, and that's where 99% of my deaths from.

Honorable mention: Patissia. Shark everywhere with Freeze and Ignite, what a joy. Imo, this boss is way more irritating to solo than any High difficuly boss.

As for party... probably Etoise. The hardest part is saving teammates from all of its fatal AoE and cheap knockback. It's not uncommon that i have to solo its last 2 phases and sometimes died from bad RNG because it evaded my CC and dash me to oblivion.

Honorable mention: Stella Ooze Hell. It's not too bad in solo but boy, isn't it truly HELL in a party....