The longest one to get was the frequent flier(play the game 25 times). This game is frustrating at first but with all the upgrades, especially the invincibility over water and the revive, make the game so much easier.
I love how most of the quests require ranged units, which are nearly impossible to acquire. I also don't understand how certain waves are fine and then go to nearly impossible to kill. I get 90% through a quest and lose because of no reason at all.
I like this evolutionary game. Great fun and multiple play styles attainable. I got lucky and went tank ... killed many of the "bosses" without any difficulty. I think I will play this game differently each time just to see the many evolution types.
Level 15 is the last decent level. I spent about 15 mins on each other after that level and ended up using guides to complete them. Puzzle games like to do this. Fun until 15 or 20 and then its the hardest game you have ever seen..... *sigh*
 "Endless Migration"){kind=icon}
 "Dino Run"){kind=icon}
 "The Tickler"){kind=icon}
 "Protector IV"){kind=icon}
 "Bubble Tanks 2"){kind=icon}
 "UpBot Goes Up"){kind=icon}