What about amiibo? I gather a fully self-trained amiibo would be a good practice opponent, since they're desgined to copy your fighting tactic...
Any insight into this would be great, I've been studying amiibo very closely, despite not hvaing one...
I think you may need an amiibo if you're going to study it as that is not exactly right. I'll admit an amiibo may be decent, but far from being the best option for practice. Mostly because they will perfect shield as they learn about you. They can't copy your tactics to the fullest and some may be completely different. They might be able to copy a couple combos but nothing really advanced.
It's better to play with a friend who plays smash (sadly I don't have one so amiibos are my only option :/) as it's better to have mind games with the human opponent rather than an amiibo who knows what you will do unless you completely change your tactics and will probably perfect shield its problems away. I should also mention that amiibos have stronger attack and defense than a normal player at level 50 even without being fed equipment.
Take a look with my Doc. One of my best,
And then he gets 2 stocked (will give props to getting a smart kill)
And as you can see, he is pretty buff. To a point where feeding him more will make him worse. He has the same stats as in those 2 videos. Over 110 attack, 16 defense, 3 speed along with those special effects, and still can't top a human player. Any unfed amiibo will absolutely get crushed by him HOWEVER, I believe the unfed amiibos are more aggressive and move more often than an powerhouse amiibo like my doc.
So this is fire mario, my best unfed amiibo
Really, he still gets wrecked by my doc but you can see that this mario moves a lot more than doc (and it's not just because doc is slower because my other mario amiibo is fed like my dr. Mario)
Anyway, I think you get the point. They can be decent for practice, but it would be highly recommend to have a friend who plays smash instead. For me, I try to make my amiibos the best against other amiibos so they should be more focused on amiibo competitions and I think they'll be better off as that(or for pulling fun stunts like a 1v7). They are definitely a lot better than a level 9 cpu. Now I don't want to sound like you should never use an amiibo, it just may be better playing against them for fun rather than relying on it to be your sparing partner.