Undyne's attacks ramp up in difficulty as her hp decreases or as time passes. In impossible mode(and also in normal mode), her attacks reach the maximum level of difficulty when she is below 1/3 hp left(or when a lot of turns pass, but usually, unless you keep skipping turns, you'll reach that part before because of her low hp). At that point you'll be facing against the hardest attacks in the game, but she also has low health, so if you manage to hit her just a few more times, you'll end the difficult part immediatly.
Viewing post in Undyne the Undying fight remake comments
It's less that I don't understand how to avoid damage and more I just find it somewhat challenging. The one that immediately comes to mind is the random spears with one of each color coming from a random direction. I often fail to spot the next one in time.
I usually get somewhat lost when all four colors are on the screen as well, but I think that's more my lack of exposure to it than anything else. All four usually don't appear until the final few attacks so it makes it a little difficult to practice.
Honestly, all of the attacks are really well balanced and defiantly avoidable. I feel like there are probably some I haven't seen yet, or at least a variation of one. I know I've only seen the 3x3 spear box once, which threw me off for a bit when it happened.
I feel somehow the same, because I have reached so far many times but there are attacks (like the 3x3 spear box you said) that I have only seen once, something that keeps me always expecting for new attacks.
I also get a little troubled when the four colours are on screen but my record is two hits remaining of hp, I just cant go beyond that point because of her evasions.
I have recorded every attemp I have made