Incredibly fast way to do this: Find chickens, unlock. Play as chicken, fly through every level until you find dragons. Kill with original archers triple shot in short order. Run and done.
For those complaining about it: if there are no ads it is either because you have an adblocker or an element of the page is somehow blocked preventing them from working correctly. Also these badges can easily be earned by playing on and off for less than three or four days. Two at the shortest
Easy way to beat this: Buy all upgrades only for the pistol and fire away holding down space, prioritizing the enemies closest to the wall. Never upgrade anything else until you have the laser rifle. Tell the other guy to use the sniper rifle. When the girl shows up have her use the freeze gun. Around that time you can upgrade the laser rifle fully in one go and some of the chaingun. From then on try to hit 3-5 zombies at a time with the laser. Do this judiciously enough and you will never even have to switch weapons, then just blast away at the boss who will go down extremely quickly.
On classic mode there seems to be a glitch or something with the mini helpers, i have two at level 6 and they all fly off the screen before even being launched then just reappear normally. This is clearly not right, hehe
Several of the math test questions definitely have incorrect answers and cannot be answered due to this. Five times over I should have completed the math test quest and could not because of that
 "Great Dungeon in the Sky")
 "Animation Throwdown")
 "Formula Racer")
 "Zombie Situation")
 "The Legend of the Golden Robot")
 "Learn to Fly 3")
 "Pingy the Ping Idle RPG")