My Gym Leader type would be Water. My team would consist of:
Gym Leader Wenter (M) (Wen-durr)
Pun-Wenter=WTR=Water
-Blastoise-Mega lv. 55
-Vaporeon lv. 54
-Gastrodon lv. 52
-Gyrados lv. 51
A Mega isn't necessary, but it would give you an extra challenge.
After Defeat (given to the player)...
The Soak Badge
TM.55 Scald
Before challenge, I say to (insert player name):
"Eh? A new challenger! Ahh, I know you! I met you earlier! So you finally decided to come here (insert player name). I've been waiting for your arrival! I've heard good things about you, especially your battling skills. Speaking of battling, you came here to challenge me to a battle huh? Well, that's great, because I love battling! Except, I'm not going easy on you!"
After challenge/defeat, I say to (insert player name):
"Ugh...that was tough. You're a tough challenger are you (insert player name)! Anyways, I'd like to present you the Soak Badge! This proves that you beaten me. Also, have this. That is TM.55, Scald. The move is pretty good! It has a 30% chance to burn the opponent, and also does some powerful damage! Anyways, now that you defeated me, go to the Pokemon League to challenge the Elite Four and the Champion, but first, you must go through the Victory Road. Get prepared. I wish you luck."