Gets repetitive fast and has no online saving. Personally, I don't understand why games without online saving are given quests?? This way, if you clean your cache or don't have it saved, you'll have to do it all over again to get to the harder quests... Also, when you start the game it won't show you on the game's page which quests you already have.
As for the hard ones, just save up the money and buy the rocket as your only upgrade, this will make them a lot easier as EvilHomer01 suggested.