i beat it on the second try. it's a matter of timing and using the metal armor tank that costs 20,000 and the super soldier that costs 150,000. if you time it right and send out like 3 tanks and then send the super soldier, at the right time, you should be able to get the soldier their untouched or barely touched and strong enough to destroy their fort. it's tricky and time consuming since it costs so much to get the soldier
the trick to NOT losing (even if it's a stalemate) is defense. buying and upgrading the guns to protect your fort will do the trick. at the highest level evolved, buy 3 of the stongest guns and nothing will be able to come close enough to touch your fort without dying first


edit: dang! just now realized there is the "impossible" level that I haven't tried yet. I'll update later once I've tried it