What are some enemies or bosses that control space, mess with the players positioning, or use their movesets more effectively in a way that makes them more challanging to fight than other bosses/enemies that do the same thing? How come some bosses/enemies are considered hard, while others are not?
Lets look at Chaos Witch Quelaag. She has a bunch of different attacks that affect different sweeps of space. For example, she can spew lava in front of her, she can spew multiple bubbles of lava at a distance. She can jump at you while doing that. She can swing a sword at you, she can attack with her legs on the sides. And she can do a large burst that hits the entire area surrounding her.
Each of these hits a different space. The sword slashes will hit the general area in front of her, but not people pressed right up against her spider head, or people around to the sides, or people too far away. Her close range lava spew covers that entire area in front of her, but not the sides or far away. Her far range lava spews and jumping attack let her attack players that are further off, as well as close the distance while attacking in the process, and all the lava spews will place lava down that the player needs to avoid until it cools off. Her leg attacks hit players trying to attack her sides and avoid her frontal attacks. Her burst attack hits the entire area around her, making it so players need to get away if they want to survive.
So in order to fight Quelaag, you need to stay mindful of what attacks cover what zones. You need to watch out for where lava is and try not to get trapped by it. You need to move in when the time is right, hang back when the time is right, and escape when the time is right. Trying to fight Quelaag when lava is nearby can mean you might get pushed into the lava, so you might not want to challenge her nearby lava and lure her away instead. The lava can also block off one of your avenues to attack her. The lava controls space in this way over an extended period of time.
Making interesting bosses is about creating variety. It’s about variety in how the boss moves, in what times they’re vulnerable and for how long, in what areas of space each of their attacks covers, in how many attacks they do each cycle, in where each attack is placed, in where the player and boss currently are when each attack happens.
I’ve been playing Breath of the Wild and there’s a lot of attack homogeneity that extends through every enemy. All weapon wielding enemies have 2 attacks, a horizontal swipe and vertical swipe (or a stab). You dodge all of these the same way and they all achieve about the same thing, control the same space. Not so interesting. Lionels are a bit more interesting with their charging attacks and fireballs, but the game could have done a lot better on this front.