amiibo effects bonuses

I used to train amiibo just bare-bones, no equipment or stat-boosts whatsoever, but now would like to try to spice them up a little. There are a couple of posts about people's favourite effects for amiibo, but not so much about the stat boosts, and the way it works is confusing to me. Please be nice if you want me to post what amiibo I'm putting bonuses on.