Interesting. So far I've tended to use lots of un-upgraded machine-guns to build a big maze and then cram upgraded machine guns and teslas through the middle paths where the helicopters run. I get to level 115 on Grasslands easy using that tactic. Now I'll try using more Teslas and Goos I guess. Haven't noticed much effect from the Goo towers so far, but maybe I just haven't used enough of them.