No, she doesn't need to concentrate on the fields for strength; she just loses them if she stops thinking about them. She can have many up at once easily. She took a punch from a VERY VERY VERY VERY VERY angry Hulk and her shield took that brunt with some painful feedback. I'd say that's some tremendous resistance to damage. Marvel describes them as being very close to unbreakable. She could also just create a field inside of Goku and then expand it until he explodes, if she felt threatened enough.