On normal, i have beaten the game with 18 super solders, And it's not too hard to!
Cheat: You can do this on any mode but, it is better if you do it on normal.
First, you need to evolve to the future. Then, you need to get the most base defenders you can get. then, get the most iron canons you can get. then, you wait
3 or 4 hours. finally, you get about 4500000 gold coins and spend all of the gold on 18 or more super solders :) .