I don't know about everyone else... but MY strategy was to grind for about 45 mins on the second level and then buy a bunch of upgrades. I run a pretty sweet gghggggghggg setup and just stun towers that will kill the lead goblin, basically just rushed and got golds on my first try every time.
I share the same complaint o basically everyone else that the getting shot from off-screen thing kind of sucks, but otherwise great game. You definitely did a great job for less than 48 hours.