Ring Attack 100% is a very challenging category. In it, there are not enough Star Characters to go around- and you'll desperately need them, seeing as how there are many inconvenient places and rough Terrains you are required to hit into in order to clear all rings.

Start by unlocking Petey in the Bonus Games. Unlocking Petey will unlock both Regular and Star Petey. Regular has a 220 Drive, which is decent in case the lack of control on Star Petey is too much for you.

Use Petey to do Unlock Character matches with both Koopa and Peach. After this, use your character of choice (a Star Character preferably) to complete Ring Attack 1P. You will unlock Regular and Star Shadow Mario afterwards.

Not only do Star Characters make Ring Attacks easier, they can also make them faster. Star Characters may even be capable of clearing rings more efficiently, meaning you sometimes won't even have to think of rings for other player characters. And for 4P Challenges, you will now have a complete foursome of Star Characters. Remember, Regular Petey is still a decent choice. Also you can use your favorite character for a 4th slot if one of those doesn't appeal to you or doesn't have fast enough hits, seeing as more than a few 4P Ring Attack challenges don't require all four players.

The only problems with this route are that unlocking Star Characters might be slow. By all means, leave your suggestions.