People don't give Roxanne enough credit when it comes to hard gym leaders. Nosepass is pretty bulky for that point in the game and doesn't have any 4x weaknesses, so even if you pick Treecko/Mudkip you aren't going to blow through it and it's backed up with an Oran Berry and two Potions, and Rock Tomb hits hard too so it's not just a wall waiting to be punched in. I think she's one of the hardest first gyms in any game.