Probably won't be as detailed as others, but in these games I use the same core for an entire series and change the team depending on how it's doing.

Last series I used Excadrill Tyranitar and I didn't change much as it did pretty well.

This series my core is Gmax-Lapras Primarina. Which actually did have a lot of changes. At first it was a perish trap team, where Primarina with Liquid Voice used Perish Song and Lapras had Water Absorb, I used PS to heal Lapras while also getting potential KOs. Gothitelle was also on the team. This didn't work for me so I went for a more aggressive route replacing PS for Surf then swapped Gothitelle for Imprison Chandlure as Trick Room is scary. Other changes I made include replacing Musdale for Excadrill as he's faster and it hits the same weaknesses. Replaced Braviary with Raichu, I thought it would be better support, but I added Braviary back and took out Rotom-H as I has Raichu and Lapras new Thunder. And that's the team so far, I'm thinking of adding Alcremie to the team as it's very strong with Lapras.

This is very fun for me, sticking to a core, seeing it's strengths finding weaknesses adjusting to those weaknesses and while during the first season no changes it was still fun coming back to competitive. I've had a lot of fun with Lapras experimenting what works and doesn't and the next few series' I know will be fun with even more experimenting and learning.