You think switching Left and Right or Up and Down for a few seconds is like learning to juggle? Clearly you never tried to juggle.
It's similar in that it's something you can't work your way through strategically, or even by relying on past experience. You have to sew it into your muscle memory from scratch. If you want to play as well against Seija's spells as you would against something normal, there's nothing you can do but work the controls into your brain. And not merely that, but you have to be able to shift into the different effects right in the middle of a run, without warming up or anything, and without accidentally reverting back to the habits you've developed for everything else in the past. It's a bizarre ability that you're not going to be good at naturally, so you have to spend a significant amount of time practicing it specifically.
But in any case that's beside the point. I'm not saying it's unfair, or too hard to learn. It's perfectly doable with enough practice. My problem with it is that it's totally detached from the gameplay itself. Your ideal reaction to it is to reach a point where it's existence is completely irrelevant, and you have nothing left but an overly simple boss battle. The "technique" you've learned is merely to manipulate the controls, and I don't see the fun in that. Learning a game's controls is an aspect of practically every game in existence, and there's probably some fun in it, but that's just a precursor to actually playing the game. If the game itself is the process of learning controls, then it has no value once you've achieved that.
That's my opinion on it anyway. I'll acknowledge that the perspective I'm looking at it from is probably not the most common. Some, if not most, people can probably just enjoy being bamboozled by it every time, but for anyone who wants to reach a point where they can say "I can almost definitely capture those spells in a full run" I think it would be tormenting.