Tax returns with original 1099's/W2s or (even easier) tax transcripts from the IRS for the past three years are the only things that are required. Technically speaking, that is ALL she is required to show. That's it.
Additional evidence of income that you *might* be asked for, since you said she isn't using assets, would be a letter from her employer on company letterhead stating her position, the length of her employment and confirming her salary and six months' worth of payslips. Again, this type of thing is usually called for when the sponsor's income is in question -- with being in the same job for six years and presumably having tax returns to back up that statement, you shouldn't have much of a problem.
Definitely don't let the forms fool you into thinking this is more complex than it really is; it's very easy to do that!