Still have to master levels 44, 45, 49 and 51 to get the Impossible badge.. It's difficult, regardless the "ghost bug" that, sincerely, I haven't experienced yet. The use of the Slow towers is key, indeed.
I just maxed the archer and owned every enemy, then started to pump up the mage and owned at double speed xD the other 3 chars stayed steady, except for some healing at the first levels. Too easy maybe? Still very nice and nice graphics.
Probably the easiest "Hard" Badge ever ^^' Very nice game concept tho :) The first time I lined the resources I was expecting the ones above them to drop down xP
For easy cash just fully upgrade Laser (and Ally at least level4) and repeat level 12 as you need. Put your ship at the center-bottom of the screen and autofire till the end of the level for 500+k :)
Custom deck, or Arch duels