Who was your favorite boss based solely on their battle mechanics thus far?
I would rank the battles like this.
1. Yvonne
2. Khulan
3. Scarlett / Lily
4. Cobra
5. Eliza
6. Karen
Yvonne's was the most I enjoyed so far thanks to her having patterns for her grabs (which I found out only recently thanks to someone on this forum) while also being unpredictable, meaning you have to remember what moves she's used while you're fighting her, but not in the "on-rails" Scarlett way
Khulan I liked her QTEs, they were challenging while not being unfair
Scarlett was too predictable, and although Lily's fight had interesting mechanics that should make it as good as Khulan's, it's way too easy for the point in the game that it's in.
Cobra's and Eliza's fights are kind of boring to me, they're the same type of "opponent charges up a move which you can identify, and have to react accordingly" fight. Bonus points to Cobra for her fight having QTEs and other interesting mechanics. If you factor in the whole stat stealing aspect of Cobra then I guess her fight should be one of the best, but I just don't like that "charging identifiable attack" mechanic.
And Karen I just personally never considered a fight, and not because you can't win, although that ends up contributing to it. What makes me consider Karen's fight some sort of glorified minigame is that your stats don't matter at all, and neither does your equipment or any elemental resistances (We don't even know what her element is, wth?), which have been key to beating the hardest fights so far. I get that this is done for dramatic effect / story purpose, but it's done at the expense of the gameplay IMO. But hey, at least it's not that bad of a minigame...