Actually , Iv beat it. All you need to do is to get to the last age and get enough money for four ion turrets . Then buy 3 super soilders. It worked for me so it should work for you.
I got a minute or 2 in the 3rd stage but died cuz they have a 4\\1 kill\\death ratio for the duelers and thats with a fire catapult. The cavemen clubmen have 4\\1 aswell without a turret but is cut inhalf with the egg auto which helps alot.
Turrets are needed to beat impossible. Always get the best turrets you can. At the very begining, dont build anything. Wait for them to have sent out several (about 4) soldiers and use the rain of fire. After that buy the egg turret.
save up for the best turrets dont try and beat the computer in the beginning just hold them off to earn money and buy the best turrents. for first stage egg turrent. fire cannon and cataplut rocket turrets and ion rays
ive beten it either modernize and try 2 keep up with the comp or stay downgraded and just by troops cheaper then the enemy would have 2 by then 4 so u can have an army ITS NOT IMPOSSIBLE!