The worst thing about Raphael is the "What's better than a devil you don't know? A devil you do" line. And it's presented as if it's clever. Awful.
That's said, I think you're being too harsh on the scene. Everybody knows that making deals with with devils never work out well; it's not going to be possible to introduce devils -- especially cartoonish dnd devils -- in a way that doesn't make them come off as used sale carsmen unless you do it in a much slower paced way. The problem to me is less in the way Raphael is presented and more in the way that the tadpole is presented -- it doesn't feel like a grace condition so you don't feel like you even have to consider taking Raph's offer. You get one little scene of your character being ill at the campsite and that's it. If they wanted to make the condition seem serious they would have to make the characters be like that one scene, except all through the day too.
I do think the entire scene would be better if he just stayed in human guise all throughout though.
