Your duo needs to be levelled-up from zero at the start of every stage, with special attacks available at levels three and five. Defeating a pranking machine usually levels up a character, so be smart about who lands the final blow.
The top of the screen shows your pranking machines as gold dots, and rival machines as green dots. Tapping the dots quickly moves the camera to a machine's location, allowing for quick repairs.
Focus all of your efforts on taking down and replacing the pranking machines first. Always pair up a Glue Gunner and a shooter with decent range. All bar one minion-type can be avoided if you stay off the pathways.