Collect 5 stars that are scattered on the course in order to charge the special attack, you can press B to use it.
Fiona jumps on your opponent' karts and gives them a karate chop. You need to drive close to your opponents in order to make her leap so activate the special attack when you are driving in the pack and multiple opponents are driving in front of you. You will need to leap on 3 different opponents in order to make the achievement pop.