This can be done once you bank 75 Jiggies. Drive up to where L.O.G. is and enter the door there. You will first need to complete the six-part L.O.G. challenge, then you will be able to battle Grunty.
- Part 1: Monster Truck - Just ram her right into the water.
- Part 2: Boat - Blast away with homing rockets and the laser.
- Part 3: Cloaked - Either method for Part 1 or Part 2 works well here.
- Part 4: Tank - Same as Part 2.
- Part 5: Broom - Take to the air and follow close on her tail, blasting away with your homing rockets.
WARNING: Make sure you get the "Burn the Witch" achievement before doing this.