First find the enemy base, then find all the ways in and out. Once you've 
done this build sandbags all the way up to the enemy base and block off the 
entrance (be carefull that you don't get too close to the entrance you've 
just blocked off or the enemy will shoot a hole in your barrier). The enemy 
harvestors can't get in to get to the refineries and they can't get out to 
harvest, therefore the enemy runs out of money and it is a lot easier to 
destroy their base. The AI is too dumb to do anything about this.