Rocket Arena

The goal of the achievement is straightforward: get 100 KOs with Mysteen.

In order to do that more efficiently, here's some tips and tricks on how to use Mysteen to get KOs.

Mysteen's rockets (standard RT shots) are her explosive playing cards which she throws fairly straight, fast, and accurately. Every third card is described to be an extra powerful blast for some reason. Plus you can hold RT down to have her keep firing, saves work on your finger. The only downside is her reload time, so be prepared to dodge while a reload is working.

You can offset the danger with Mysteen's shield ability (LT). The energy shield will block some attacks but will only stay up while Mysteen is not attacking. So if you're holding down the shoot button, you'll probably only have the shield last a second or so instead of the 3 seconds, I think, it would usually provide.

Mysteen is a master illusionist with her copy ability (RB). You can fool the opponent by switching between your copy at will with RB (check to see RB is there on the screen, otherwise not being there would mean your double got KO'd or timed out).
The power comes in the fact that the double copies every move you make Mysteen do, meaning you can provide double the firepower when attacking.
The trick then is to split up right before you attack someone and keep shooting, no matter what. Even if your Mysteen doesn't have a target, Mysteen's double might. I've gotten quite a few KOs from Mysteen's double's shots as well as Mysteen herself. Make use of it.
If you find your active self close to blasted (aka full blast meter) press RB to switch over if the prompt is still there to see if your double fares any better.
For instance, your double can also be a lifeline if you're knocked over the edge of a KO zone. Press RB and teleport over, saving yourself from a knockout.

Note that if you're in a Rocketball match, Mysteen can't use her double while holding the ball.
Also in Megarocket, be aware that only the real Mysteen can hold a megarocket point.

Mysteen also runs slowly. Use lightning powerups to speed her up and dodge bandanas to keep her elusive.

Have fun!