Man or Monster Game don't have cheats, but there are some tips that can be helpfull for you.
If you play with the man: First and most important tip is to save money and weapons for next mission if you don't need them. if you see that monster will be destroyed don't put more tanks or any weapons because you will lose them for the next mission. every next mission is harder, so don't waste weapons. Try to plan the defence to destroy monster and put your weapons on the places when you think they will not be destroyed. Another tip that can be usefull for you is to shot monster from distance and dont go to close to him because he can easy kill you.
If you play with the monster: There are not many tips that i can tell you, but main tip is to destroy everything. Try to organise attack. First destroy all weapons that can kill you and then destroy whole city. Buy attacks that you can shot on distance so you can faster destroy all weapons in city.