Phone repair shop owner BROKE my iphone 6plus! Legal

The store owner opened my iphone up and dropped it HARD right on the ground, The phone lost power and will not turn on now! Shattered glass was all on the floor! This owner says DROPPING IT WON't BREAK IT! And other bullcrap.
I have a witness and a apple care repair specialist as a witness willing to help because he seen my phone working perfectly fine 20 mins prior to this incident. Both apple store and repair store were both in the mall! I told security and they made a record of EVERYTHING
The store owner is not trying to help me but is trying to get away with this. He turned a $100 repair into a $800 repair. My iphone is still under warranty with apple.
WHAT CAN I DO?!

I would tell him he needs to pay for the damages done, otherwise you will file it in small claims court (you don't need a lawyer). If he doesn't then file a claim against him, keep all your paperwork as proof and pics of the phone etc. You will both be required to show up for the hearing, and if one of you doesn't show its an automatic loss.

Thats what I would do. As a side note this is why I would go to an Apple store to get it fixed, they will stand behind their work no matter what happens.