hey, so I have never contributed anything and figures might as well try, anyway, you got a lot of array of mons right there, gotta clap to your dedication on training on each of them.
as for tips on how to play through Reborn with core pokemon rather than training every single one you have is to plan early on your team, it's like playing a competitive battle.
my pivot for making a team in Reborn and similar fan pokemon game are as follows:
- Your team has to cover various role in battling (example: sweeper, tank, status setter, etc.)
- It covers various type, try not to have a team that consists of pokemon that only effective to one or two types (don't worry, Reborn gym leaders usually have secondary type in each of their pokemon that you can use of).
- High stats for the pokemon role (e.g. sweeper must have high speed and attack stats), you can check stats for every pokemon at bulbapedia.
- Have a good moveset to play their respective role.
- You like the pokemon, so you wouldn't be bored while you have to grind them.
as to how to play just with core pokemon, try to utilize:
- Field Effect, use em or destroy em (just destroy Serra's field, Mirror Arena is a hax.)
- Your pokemon role, (e.g. unleash the speedy stealth rock setter, then SWEEEP SWEEEP with the sweeper with priority moves.)
- Gym Leaders pokemon's secondary typing, just because Corey is a poison Gym Leader doesn't mean Mareanie is not affected by a good ol' charge beam.
- Consumable and niche items, those revives, air balloons, etc. can come in handy.
- EV training, if over-levelled, common candy to lower them.
Lastly always have 2 or 3 backups in ready to alternate your strategy or to give that whooping a little bit more wham when your main core team can't defeat the Gym Leader (or trainer, or that stupid hooded Admin) no matter what strategy you use
these are my personal guideline to decide most of what I do in Reborn to ensure a win, hope it would help you in your future endeavour, Good Luck!