Also, Ibis and Balteus are widely considered the two hardest bosses in the game and I think the common denominator is that they’re the only two bosses which actually retreat. The AI of every other enemy is always aggro’d and mindlessly pursues you, but these two are scripted to move far away from you regardless of what you’re doing, which I believe is what throws people off. Because of this they’re the only enemies that feel like PvP opponents: You can be about to stagger one and then it will suddenly turn tail and get out of weapon range, etc.
I tried to play as much of the game under-leveled as possible and fought Balteus for the first time with parts you either start with or unlock automatically up to that point. He was the first thing in the game that filtered me, until I swapped out for the sword and used it aggressively, at which point everything fell into place. It made me realize that there’s basically no disincentive to hugging PvC enemies as tight as possible. Even if they have melee weapons or dodge-able projectiles with extremely fast velocity, they’re always disadvantaged by close proximity more than you are.