Funnily enough when I was planning the battles and how each person would win ages ago, half the ideas were from watching Initial D, a drift racing anime.
Justin's tactic was inspired by a win where during the practise runs and the first three quarters of the race Takumi (the protagonist) didn't let his engine rev past 7000rpm, when really it could rev up to 11,000, and using that difference only when it would win him the race.
Charlotte's battle was inspired by a race where the challenger was racing on a course not his home course, however the mountain that was his home course was only one way roads; one going up the mountain and the other going down, unlike the standard single two way road. As such, when he raced on two way roads, and the corner around the mountain went to the right, he'd take the corner wider, to account for the possibility of a car coming up the mountain in the opposite direction, allowing the person he was racing, who was more experienced and knew a car wouldn't be coming around, to take the win.
I had another tactic in mind planned for Abbee and Victini, effectively creating a long drawn out match and waiting for the perfect opportunity to end it, however it didn't suit the matchup (Victini vs Shaymin) so decided to use some nostalgia from S.S. Wishmaker to up the stakes of it, as well as show Victini's own strength growing.
As for Jon's, my original plan wasn't to have him beat Camilla with just Blaziken, however I was sold on the tactic against Suicine and it was going to be near impossible for Camilla to counter it any different. Whilst I could have had Raichu or Skarmory finish Blaziken, the rest of the match would have been too easy and low stakes. So I'm glad it worked out that way.
I spent far too long finding a Pokemon that has low enough defence and HP that Blaziken could beat it with a single quick attack, that also knew Quick Attack itself to make that work.
In terms of planning, it has become much more meticulous than when I started. Season 1 it was writing as I went. Now, it is two to three stages. First is writing the battle roughly speaking, with major moves referenced. Then it is written turn by turn using Showdown's Damage calculator to keep it realistic, then actually written into the story.
Funnily enough half the reason I am having them do as well as they are is Willows observations from Spiritwater, about the fame and renown of the Academy. I had only just started season 2 when you posted that and I was like "okay I have until Spiritwater to make them huge".
Speaking of Spiritwater, there are some things that have happened in that that haven't happened yet, so you may be able to figure out what the Epilogues for Season 4 may be... Planning for the first is done. Now I just need a moment to write...
Bookmarks