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Final Gym, Steel-type. All of the regular trainers in the Gym are scientists and they're doing research on different battling formats. In order to challenge the leader, challengers have to beat five scientists who all specialize in a different format: Double, Triple, Rotation, Inverse, and Sky Battles. I would challenge the trainers in a Single Battle with a weather team (pic related, EVs and natures can be ignored). No puzzles because puzzles are shit, only battling.
Badge would be Stainless Badge. Victorious trainers are allowed to choose between TM91 (Flash Cannon) and TM60 (Iron Head, which now replaces Quash because nobody uses that move). The second TM is given after a victory in rematch.
Pre-battle:
>"So, it seems you have mastered all different types of battling. Let's find out if you're prepared to face my team."
After being defeated:
>"Impressive! I expected nothing less from an expert like you. Take this Stainless Badge to silence anyone who doubts your skills."
>"I would also like to give you either one of my two favorite TMs: Flash Cannon or Iron Head. They're both equally powerful and accurate Steel-type moves but one is for special attackers and the other for physical attackers. Choose whichever you like more."
>"If you want to get the other TM too, come challenge me in a rematch. Not until you're stronger though, I want you to be the Pokemon League Champion the next time we face against each others."