I probably played this game wrong :D
Mass towers made me invincible, and because he kept sending units I was getting more scrap than I can spend, because I was spending all oil on trucks at first - I had better economy and because he sent more units (more scrap for me) - I also had better defenses, then I simply spammed both - tanks and soldiers equally and upgraded only their armor - again equally. As for research I was spending all points into cannon damage (obviously). By the end I had so low framerate that I couldn't build towers anymore (browsers probably couldn't catch my mouse presses due to low framerate).
Edit: tower cost must all rise with their number, otherwise they're very cost efficient.