SpeedRunners

This achievement is super easy. Grab the rocket from the blue box (it is random what you get so just keep grabbing them). When you have the rocket in hand go up to a wall or jump towards a ceiling and blow yourself up with the button. It is as simple as that. If you are having trouble here is a video guide.

Guia de conquistas - Tiro pela culatra - Comentários em português do Brasil.
Guide Achievements - Backfired.

First, you must obtain a rocket by running into one of the bluish item bubbles around the trails (the items are random so this may take a little while to even get one). Once you have the rocket I suggest waiting until it is a showdown between you and one other racer that way you know who the rocket is going to target (although I do believe it targets the closest racer to you). I also recommend firing the rocket in an open space so it doesn't just blow up on a wall or ceiling. Once more I suggest being in front of the other racer as it is easy to fall back and catch the rocket than it is to catch up to the other racer in front. When you have the rocket in your inventory simply press to use it.