The guide you linked is only for getting all the minikits, not completing the game. If you are stuck in story mode, this will not help you at all. If you actually are trying to get the minikit, you'll have to go into the level in Free Play mode and use a character with a gatling laser, such as Hevy. If you don't have anyone with a gatling laser, you'll need to unlock and possibly purchase one first. I believe just completing the Story Mode of the level Rookies will automatically unlock Hevy.
For future reference, the site that Draykov linked, GameFAQs
, is the best resource I've found for guides, cheats, tips, discussion, &c.