Create a custom match against one skilled opponent. Any of the maps will work, however, I like Pioneer's Peninsula. Whichever map you choose, you're going to want to go first. If the computer has the first turn, restart the match.
The supporting funds can be found in the top left corner. They begin at "10", so if you see them go above that, restart. They increase as more tiles are destroyed. Therefore, building more than one or two harvesters is not advised.
An infantry rush works well for this achievement. First turn, build an armory on the tile closest to the opponent. Advance your walkers towards the enemy's base. Each turn, build the maximum amount of walkers and continue to advance. Try to trap the enemy within their base. If they expand and start building harvesters, your chances of getting the achievement drop dramatically. Build a second armory when you're six tiles away from your initial armory and continue to buy walkers.
If you need some quick funds to make the final push into your opponents base, you can build a harvester. Build it on the lowest tile you own, to minimize the damage it does. If you win before the supporting funds increase, the achievement is yours.
Taken from Shelton's guide on xbox360achievements.org.