This is really hard in a regular race. It is SUPER EASY in Arena Mode. The achievement says "In a Single Race", however it works in Arena Mode.
Simply start up the "Kids Choice Awards Arena". You can do a circle lap around the center. Hit the 4 ramps in the circle. Make sure everytime you jump, press
to execute a Slime Stunt. Do this 25 times for a Easy Achievement.
VIDEO GUIDE:ACHIEVEMENT BREAKDOWN: