A very interesting Concept!
I must say this game could provide me with hours of fun!...if it wasn't for one problem...
around level 3, they start getting really sparse and I don't understand why...combos became nearly impossible everytime I tried, and anytime I purchased the miners they came at least 2-3 seconds after the last chopper and 2-3 seconds before the next...making them useless except for free hits...
I don't know, but maybe a little algorithm to make sure there are no big gaps in enemies might be helpful...I should really be able to take my hands away from the keyboard for more than 3 seconds without consequences.