Damn, I wonder how people are getting such high scores. After round 70 or so, it becomes incredibly difficult to kill those enemies even with second upgrades. Maybe you have to upgrade to level 3 by then, but how to get enough gold then?
This is just my opinion, but those who are on the highest score must work together with each other, they share strategies while looking for better ones and try new things, this is not an act of cheating.