Slam Dunk is pretty luck dependant. Some people have said shooting the fire extinguishers she stands beside works fine.
Didnt work for me so I had to mess about with fire extinguishers, napoleon, and remote CX. Got it in the end after an hour of trial and error.
If she gets stuck in the underneath of the canopy, the challenge seems to pop. Maybe try and put her body directly underneath it, put enough explosives under her body, and hope the force will make her get stuck. ICA Space Program may help with this.