Moongolow help!

Hello, me and my husband are trying to get Yellow Sign. We looked it up and it says we need to go to Moongolow and use a standard character and after 14/15 minutes, the boss will show up and we can go to the bonus area/level. But every time we do this, it just ends the level and death gets us. I don't get what we're doing wrong? We did the same thing that a walkthrough on IGN said but still couldn't do it?

I would definitely appreciate the help because we want to make it passed this area lol Thanks :)