How to beat challenge mode: 15 upgrades on damage, then starts leveling only fire rate. DO NOT spent points on speed. Also, in the begining of the game is acceptable receiving some damage to conquer flags.
I don't undertand people complaining the trials are impossible. I just think they are ridiculously easy cause I finished ALL them in the first tentative. My units: Vampire Warrior, Goblin, Elf Hunter, Elderly Wizard, Amazon, Centaurion. The equips are all from the page 1 (junk). Just let your heroe in the left size and attack with the others. Ocasionally cast spells to damage the bosses and heal the heroe when the boss is to close. Sometimes you need to move the heroe to the right size, so the boss will follow him defenseless being hit by the long range units. Hope have helped and sorry about my english.
U can get the easy, medium and hard badges using the mouse, but for the impossible u need to practice with the keyboard. Try this way: control the 3 top buttons with the right hand and the others with the left hand; right hand -> fourth: 9 , middle: I , index: J; left hand -> middle : V , index: space. Try mirror for left handers. The real problem is the Eight Circles song, but with little practice is possible. Remember that when u miss a note, u percentage is also decreased. So, don't get upset if u can't do more than 85% because when u master a few
more notes in addition to not lose percentage u will win some, which will rapidly increase ur percentage to 90%. Sorry about my english, is not perfect.
 "Idle Mine EX 2"){kind=icon}
 "Sugar, sugar"){kind=icon}
 "Solarmax 2"){kind=icon}
 "Bunny Charm 1.2"){kind=icon}
 "Ultimate Santa Battle"){kind=icon}
 "Edmus"){kind=icon}
 "William and Sly 2"){kind=icon}
 "Prizma Puzzle Challenges"){kind=icon}
 "Epic War 5"){kind=icon}
 "Swords & Potions"){kind=icon}
 "Symphonic TD"){kind=icon}
 "6 Differences"){kind=icon}
 "Step Seq."){kind=icon}
 "The Trader of Stories"){kind=icon}
 "Flagstaff: Chapter One"){kind=icon}
 "DJManiax"){kind=icon}