QA should come up with test case/script based on spec.

QA is not supposed to trust Dev, but rather inspect what you do, according to the specs, NOT what you implemented.

So, you should never tell them what to test. It's their job to work with BA and/or PM, or even end users, to come up test cases and test scripts to cover ALL features in the system.

Isn't that what check and balance is about?:)