Perhaps I haven't noticed this because my amiibo has a really hard time KOing me.
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On a separate note, when it came to training my Rosalina & Luma amiibo I did a mix of things when trying to teach her, like using items early on, trying different stages with different mechanics, and more, but I did a few mistakes (some of which were major in themselves).
First, I didn't introduce her to the swimming mechanic early on. Whenever she got near the water, she immediately recovered with her special. Because of this, she could be punished easily, however, when she did go in the water she learned to swim and jump out, without fear of drowning (she does steer clear from it though).
Second, I used some custom equipment and specials on myself, but not her. This mistake was very minor since she could easily adapt to what I threw at her.
Finally, my most terrible mistake, was not introducing her to other fighters. I focused too much on teaching her new material and not letting her use what I taught her against other fighters. She only fought me, Rosalina, and know she knows very well how to handle herself against any other Rosalina, but now that she's at level 50 and hasn't fought every character, she isn't able to adapt very well to fighting new characters.