S = Stereotypes.
It could be argued fairytales have a lot of stereotypes in them. The Big Bad Wolf represents villainy and indeed the saying has passed into the language. We say someone is known to be "a big, bad wolf". The downtrodden types that have their lives turned around for the better are known as Cinderella types. I think what any fairytale writer should do is use the tropes wisely but not be confined by them. What does your Cinderella type do to try to help herself/himself out of the situation that they're in? Maybe it is that which attracts the attention of the fairy godmother to help them in the first place. Stereotypes can also be spoofed or reversed as in the Shrek series. So use stereotypes, they can be a useful shorthand, but put your own stamp on the characters you are creating so they are clearly "their own people".
T = Tricks
It is fine to use stereotypes to create shorthands for your characters, who should then still go on to be characters that are uniquely your creation, and other writing techniques to improve what you do, but those should be the only "tricks" played in your stories. Indeed they shouldn't even show! Your stories should read "naturally" with nothing drawn to the reader's attention any "artificial devices" have been used in the making of that story. As for tricks played by characters on others, there should be ground rules set out early on in your story as to magical capabilities so readers know that character A could be reasonably expected to play such a trick on character B.
U = Unicorns (and other mythical beasts)
Use sparingly if at all! For me a story is all about the characters. Unless you are writing a story from the viewpoint of the unicorn or other strange creature, there seems to be little use for these, other than as transport, possibly, or to set the scene for how your world works and looks.