An easy way to gain this achievement is in local co-op with two controllers. Set the Toy Box to 100 and the money to $10,000. Set up one turrent on each side.
Using the main profile press 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. Press to use it.
An easier way to gain this achievement is to start the game, press , try to get the Napalm, and if you don't get it to just surrender and restart. Repeat until you get it.