Obviously it was a one-sided slaughter, but they very clearly established both in and out of fiction that they were deliberately trying to do that from the outset, so that's fine. This episode wasn't about the result.
So why would Deadpool know about Bighead's weakness about removing the mask? He's aware of events outside his comic's contained universe, but he's not, like, freaking omnipotent. Like, he doesn't even know everything that is everything in Marvel, just being able to communicate and interact with the writers of his comic doesn't mean he's an expert on reading Dark Horse comics. It's a strange leap in logic I've seen applied a few times and I'm confused why? The joke should be enough that he breaks the fourth wall, he's not also all-knowing
You could say that because Lobo managed to figure out how to deal with it it's not impossible for Deadpool to have intuited what he had to do. But given everything else in this episode it would be totally justifiable to say he knew Big Head's weakness because he heard Wiz and Boomstick talking about it a few minutes earlier.