I think, its still easy. Once you found the right strategy. (Sniper first, then tesla) And then it 's just bulding some more, hit fast forward and auto waves, waiting for about 15.000-20.000 bucks.
But the start of each level is finding the right place for your first sniper and tesla, the shotgun or nomal towers are useless and just a waste of money. Maybe you should start each level a little bit easier, i.e. less HP and slower enemies. Then build up. And at the end: FULL MAYHEM. I only use sniper, tesla (a lot), heat ray and rockets. Btw, red enemies seem to be less affected by bullets, black/green by tesla. But still a fun TD game.
 "Use Boxmen"){kind=icon}
 "Frontline Defense Beta 2"){kind=icon}
 "Arachnid Wars 1.5"){kind=icon}
 "Music Catch 2"){kind=icon}
 "Fantastic Contraption"){kind=icon}