There was a strategy posted for the impossible level that I found helpful. You stay in the stone age for a long time, you just keep building the $15 stone age melee guys. I stayed in the stone age right up to when the computer got to the 4th age. If you have like 10 guys stacked on top of each other, they attack in unison and can kill the army guys easily. Once you get enough EXP and gold you upgrade right to the 4th age and build double turrets.