Knockout City

Each map has a chance of spawning a special ball (the intro to a match will show which special ball it's using), but not every map can spawn each type of ball AFAIK. When you get a Cage Ball and hit someone with it, they'll be forced into ball form and you can pick them up and throw them but they will break out of the ball after a time.

All you need to do is pick up a caged player (you'll know they're caged because they have a cage around them, when they're in normal ball form they don't) and toss them off the map. They need to get KO'd while in the cage, so if they have little time left on their cage timer you likely won't get this. So picking someone up as soon as you can after they get caged and trying to make sure it's close to an edge of a map will help a lot with this, as will throwing them straight down off the map (just run over to the edge, look down, and do a charged throw to throw them faster).