Getting Expert on missions is simple, if you pretend this isn't a tower defense game. Max out all bomb upgrades, starting cash and money per kill. Place one super tower towards the end of the map, hold "N" down to summon all waves instantly and bomb the heck out of them. You can hold shift and just keep clicking to keep bombing. I think it's the only way to Expert the higher levels and it's pretty lame if you ask me. You also might break your mouse on the last few because you have to click as fast as your finger possibly can.
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