An instructor of monism can't possibly exist. Does he know the truth or not? The first can't be, for knowing the truth there is no second soul for him, there would be no student for him. The second also can't be, for it is not possible that ignorant person may be an instructor.
Let me explain the argument.
If a person speaks about Advaita, he doesn't know the truth, else he won't instruct since there is no second soul for him having realized everything is brahman.
The same applies to Krishna and all the mantra drastas in the Vedas. Do they know advaita? Then they won't instruct. If we accept them as perfect teachers of truth, then advaita is false on the basis of scriptures since they actually instructed.
How do Advaitins defend against this argument?