If you're having trouble with this game, I suggest this. First, Max out your Science skill speed. Then, put points into Money gained from killing monsters, accuracy, and health Regen, to rank three. Next, Max out Reload speed Then max Health Regen and armor, You see what I'm getting at. Tree research is the last skill to research. Only start work on the tree once you're maxed out. I reccomend buying the Pistol, then the machine gun then the heavy.