Here's what kinda strikes me as "random" in these fights.

Hull Isle: Random people get thrown, inked, and chosen to spawn exploding water tornadoes.

Stone Vigil: Last boss has no aggro table. He turns and does a kind of emote... If he leans back and kinda breathes in, he'll do a mid-range fireball. Melee is safe, max range is safe.
If he leans forward, then he'll charge in that direction. And barring those 2, I think the only other thing he does is turn towards someone, or in some direction, and do a melee range frontal swipe... So its all "random", but telegraphed.

TTD: Um, random line attacks, random ground aoes... But they do ~500 damage each. You'd need to literally stand in the middle of ALL of the line attacks to die.