There is always and easiest way to beat challenges-
Racing/Get to the Flags: You can be your favorite character, but the best chice is be the fastest person you have currently-Top speed and acceleration.. Also, if you cant beat it then, try taking time on some retrys looking for where the rivals go, and ways that are faster. Or ways with lot of graffiti so you can get boosts.
Tag fights: Again, best bet being someone who has the stats to fit this. Choose someone who has high stamina and graffiti. Now all you have to do is get behind the Rivals. If they are spit up or not, they will take a repeating lap. So once you figure it out, just wait, and when you get the chance, go. But dont waste your graffiti on boosts. Its not worth it then since your attack wont work.