Get +money and +luck to start,
wait for level 5 laser wingman,
the next level you will have enough to buy another lvl5 upg (get reload and damage+), then get armor and game is over from there.
The level 5 boss is easy if you use your mouse :).
Loved this game, but won't try for impossible!
for the medium 20 just build lvl1 fire guys until the map is full (without paving), then upgrade all together. Same for medium 15. Put your skill points in fire and the game beats itself.
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 "Protector"){kind=icon}