2. Better, Faster, Stronger
Once you've acquired your beefy units like the Sherman Tank (you can be creative if you like--just make sure it fits), it's time to give them the shot in the arm they need. So, head on over to the Army Research Lab and train the Sherman Tanks in Accuracy, which increases the chance of Critical Hits. (A damage or strength upgrade wouldn't hurt either.)
This is vital to the process, because Critical Hits are proven to increase the amount of resources that an enemy drops in battle. And this is the sole reason for this strategy, the power of Critical Hits.
3. To Battle!
Now, it's time to try out your Sherman Tanks and blow up some planes. The 7th battle in the Mercadia 1 series features two fighter class Air units and two bomber class Air units. So, bring at least three of the Sherman Tanks along, and one pumped up unit to take care of the bombers. Even a basic, upgrade Cannon Artillery unit would do just fine.
Then, select the right targets and let the Critical Hits roll in, dropping all sorts of free resources and Energy to compensate for the Energy spent. Remember to have your Sherman Tanks and Cannon Artillery eventually upgraded to the max, because the lynch pin of this strategy is to lose as few units as possible in order to maximize your gains.
Rinse and repeat this until you have a load of new Oil, Wood, Ore, XP and Coins--and don't forget to keep that Bonus Bar going for extra Coins. Be sure to take advantage of this cheat before Zynga catches eye, as some players believe it already has, though nothing has been confirmed