Offering the statement of a party not testifying to prove the truth of the statement is called hearsay and is not admissable in court.

I can't testify specifically about his real estate courses.

I was trained by excellent mentors & had to testify many times when something went South. 

Oh wait,  I went to college and law school also,  AND I have the added education from the Appraisal Institute and certification from the state of California to testify as an EXPERT witness in real estate valuation matters! 

I've also been in court many times to testify in criminal cases relating to my rentals (including yesterday and tomorrow).

Will the neighbor go in and testify?  

Having been one recently I can testify that they are likely sleep deprived, anxious over kids sniffles, and exhausted from moving in.Are they concerned about lead or other contaminants?

Your only option is to file a N5 (reasonable enjoyment) keeping in mind if the upstairs tenant is not prepared to testify at the hearing you will be wasting your time doing anything.

Not too many people are testifying how it helped them make money.

If you don't have a recent arms length transaction, or the hearing is expected to be contested because you're asking for a large reduction in value, you can hire an appraiser to prepare an assessment of value and testify on your behalf.