1) Rule 1 - It is a Shadow Monster. Black and gray is all you will be allowed!
2) If it is a boss type shadow beast, make sure it has a justifiable feature that gives it an edge over other shadowy creatures. Bigger does not necessarily make it stronger.Also, creatures born of shadow do not have glowing eyes or body parts for that matter, unless used for intimidation. They'll get spotted easily by the other shadowy monsters out there.
3) Make sure the boss' species suits the minions, unless enslavement method is used. Should there be a diversity in minions, give the boss a typing which gives an edge over ALL the minions.
4) Never EVER give bosses personality. Insanity streak like Kefka, yes. But if I ever catch you portraying a shadow boss in a lighthearted manner, you will pay.
5) Remember. Bosses are manipulative, otherwise they have no right to command minions! If you do have to make one which is just plain powerful but not manipulative, at least give it an assistant who can command an army and is manipulative.... who'll stab it in the back in the end!
6) Give the boss a reference point for its features to be recognisable. Even blobs have to be recognisable as a blob.
7) There are 2 kinds of fear - Tangible "Physical" fear and the Intangible "Abstract" fear. Make sure yous Shadow Beast has both fear factors.