Does anybody have tutorials on how to beat gym leaders on hard mode?

Currently playing nuzlocke hardmode and i cant beat the first gym leader without my pokemon dying. I got rain dance to take care of sunny day but its not enough his pokemon still do so much.

I just trained my pokemon like hell, the easiest and noobiest way. Killing Rotom over and over is a good way to train if that’s what you need

But you cant level them higher then the level of the gym leader, where can kill rotom?

in any house you can interact with the TV and you have a chance to encounter Rotom, just keep interacting with the TV and you’ll keep fighting Rotom