At first I was waiting a few seconds to spam the normal units. Then, once I had enough money I spammed the fast units. This was very quick as every time I was putting like 10 fast units, I was getting an insane amount of money. I did this thing like 5 or 6 times until I started spamming the high hp ones. At the end every 5 seconds I was losing so much money that I went in negative :)