Can be done in local versus with two controllers. Set the Toy Box to 100 and the money to 10,000. Set up a couple of turrets for both sides.
With the profile you want the achievement, press X and select the bottom option. If you don't get Napalm, spam the cheap armoured cars from the other team, and kill them with your commmando to build up $8k and then try again. Repeat until you get a napalm strike, then press Y to use it.
Cringer 85 suggests the fastest way is to start the game, press X, try to get the Napalm, and if you don't get it to just surrender and restart. Repeat until you get it.