LOL i started the first level but didnt fire my first shot right away, well when i came back and fired my shot the castle was already collapsed. strange :S
The way I beat challenge 6 was to break down a single ball until you get more ammo, then collect the balls on one spot so that when you fire it, you get something of a "shotgun" blast firing in one direction. Get close to a larger ball and it will split it and many of the smaller ones within it. Hope this helps.
easy have 3x ghosts in the back and 3x trikes in the front, try to max lvls and get upgrades, you'll be hitting with all your animals almost every turn, so its efficient. plus the trikes deal a lot of damage, and when all are maxed out, you're hitting twice with the trikes at 11 dmg each and three times with the ghosts at 6 dmg each.
 "Dead Zed 2"){kind=icon}
 "Crush the Castle"){kind=icon}
 "Oozing Forever"){kind=icon}
 "GlueFO 2.0"){kind=icon}
 "Spot the 25 Differences - Challenge 1"){kind=icon}
 "Brute Wars"){kind=icon}
 "- Music in Motion -"){kind=icon}
 "Castle Crashing \"The Beard\""){kind=icon}