Just a quick and short solution:
Load level 1-2 of the new DLC and put the chef you want to fire in the cannon, the player who is going for the achievement has to fire the button. You can do all 100 shots in the level, because the timer don't start until you serve a meal. Don't worry if you see no tracker progress, it will refresh at the end of the level OR your achievement will just pop if you reach the requirements.