OK, three tips
#1 THE LEVEL DOES NOT MATTER
I've seen level 30 amiibo annihilate level 50 amiibo, because my friend wanted to quickly level it up, he set it to fight level 6 CPU at 99 stock, an went to work. Good idea, but like school, numbers aren't everything.
#2 Train it using a variety of characters
Let's say you use your amiibo exclusively against Zelda, this level 50 amiibo would easily decimate any Zelda.
But what happens when your amiibo fights another, completely different character, like Little Mac? The amiibo wouldn't
be able to handle Little Mac's attacks, and would get beaten quite easily by him.
#3 Keep fighting
Like my Sensei once told me, until you're the best, there is no rest. Keep playing with your amiibo, even at level 50, it's still growing, not as fast as a level 30 or 20 amiibo, but it still has room for improvement.