I've just read that overkill damage counts for "250000 damage" badge. This is important. That means I only need to get max waves and double enemies per wave, and I don't need to double their health (which I did every time, and each time got slaughtered). Once this badge is gone, everything else is easy.
It says that you SPEND money and experience when you upgrade an unit. However, when I upgrade an unit I get EXTRA experience. Is that a bug?
And does it make a difference in experience of leaders if you upgrade their units or not?