[pigeonpie]

Luckily my girls chase any cats off and if they haven't noticed it then opening the door whilst shouting "cats" at the dog usually sees the cat making a hasty retreat (usually followed by one labrador and 20+ chickens who have decided to see what all the fuss is about and regularly follow the dog).
A good aim with a water pistol may help deter your bully cat, otherwise the only other thing I can suggest is using electric fencing to keep the chickens bother free.  We used to have the problem of cats messing in the veg garden (before we had the chickens) so we put some electric fencing around it and sure enough the problem stopped and even now the cats avoid the area, even though the fence has been down for years.  A shock from a domestic fencing unit is enough to deter a cat but not hurt them.

[Lindeggs]

... its hard to make chickens scary, they are just too sweet!!

I dunno about that, CC!  I've seen some pretty scary chickens in my time! 

Of course if your chickens aren't scary enough, you could always adopt a goose or a swan and let it hatch some eggs.  Those mad birds would attack a lion if they thought it was going to mess with their babies!