I'll second using electrike. Don't send it out until corey sends out crobat so his venoshock is only half power and thunder wave it so it's less of a threat.
I've really enjoyed using a super luck absol with a scope lens. It guarantees crits on moves like night slash and psycho cut. Add swords dance for boosts and sucker punch for priority and it hits like a truck.
I hope he does have a tyranitar to boost his special defense of all his pokemon. Paired with weakened water type attacks from the dragon field, special water moves would be nearly useless.
There was some good speculation in this thread if your curious what others have said about the voice:
http://www.pokemonreborn.com/forum/index.php?showtopic=17363
I think a guy sells them for 10,000 when you first get to the grand staircase, but he's only there temporarily so depending on where in the story you are you may not be able to buy them.
That's not a headbuttable tree. The ones you want will be purple. I know 2 of them can be reached by going through the grotto. You enter by jumping that ledge in your screen shot and entering the building immediately on the left and make your way through the tunnels. I recommend you bring flash.