Shield is useless.
...lol
Blocking is instinctive in the sense that you shouldn't be mashing all day to get hit by random shit. Again with mashing for combos, hit buttons with purpose. Otherwise you may/can/will get the wrong stuff occasionally when you don't want it, and when you do that with her you generally die. When playing Nero, usually you want to play safe enough not to get hit by summons/random 4c. Wait for him to mess up, get the knockdown, and lock him in the corner. Bunker cancels pull you out of blockstun. So it is: You getting hit -> shield bunker into whatever you are canceling it into. She doesn't really have many good cancel options so I wouldn't really worry about it.
Arcade mode should be cake, comps don't block. =<