When unlocking characters, you will most likely lose to some of their challenges. In the old game you had to clear all of classic mode in order to fight them again. Thankfully in this game you can fight them again without too much trouble. You will find them right where you left them. Just keep doing Vs mode battles and you will get to fight them over and over until you manage to beat them. It may take a couple tries to learn their weaknesses, find the right character to fight them with, and sometimes to overcome some really bad luck. So don't stress too hard over it if you don't get them the first time. There's always more chances to fight them.
Every man dies, but few truly live. Live your life to its fullest, every day as if it were your last.