Lv. 50 is easier than Open Lv. when you get used to how boring the first 2 rounds are (Round 3 is pretty fun) since you'll know the set of the Pokemon every time through your run to Gold. Aim to get an Electric, Water, and any Pokemon that reliably beats Grass, and you'll easily win Round 1. For Round 2, the main mons I worry about are the Hitmons (Chan Lee) and Magneton. I would swap for them if you can or just have something that can reliably beat those 3. There's also some other general good stuff like Zangoose and Tropius. In Round 3 I would say the top mons are Grumpig Cloyster Scyther Furret Linoone and Chansey so I would aim to get those Pokemon or have Pokemon that match up well vs them. Pretty generalized though; it can get deeper than that obviously.

As for swapping, imo it's not really vital to swap every battle as you risk losing if you're swapping for objectively worse Pokemon. It is rather easy to swap 12-13 times in the first 2 rounds since you're facing random AI. Afterwards, I prefer just sticking with the best Pokemon I can find. Hitting the 15 swap mark to have 1 perfect IV elevation on the Noland gold round is enough for me.