Argument:

If none of our better teachers are tenured, then the tenured teachers are all very politically conservative,(premise)

And our better teachers will be conservative, (premise)

Therefore, all of our better teachers will be tenured.(conclusion )

Attack:

If this were a well-formed, structurally sound argument, a false conclusion could not follow from the true premises. But since one of our best teachers is tenured by other ways, the conclusion is obviously false. Teacher who is politically conservative is not the only way to be tenured. Although teacher who is politically conservative can be a necessary way for being tenured, the tenured teachers were not just by this way. So its a flawed argument.