Edit: wow, my phone did a number on that. I've rewritten that paragraph as well as writing my own "balance" notes.
When you're developing a game, one of your first instincts is to tell the potential beta testers that you're not looking for anyone to test the game, because they give off that vibe that they just want to play the game early. A good beta tester will test your game with a goal, such as balance testing, which is your best bet for what you're trying to do, or just attempting to 'break' the game(either by finding ways to make it impossibly hard or disappointingly easy or by finding game crashing bugs). While there are a lot of people who volunteer to beta test here who do just want to play your game, I recommend finding a dedicated playtester who demonstrates the ability to document what they do and if at all possible has some design insight of their own for gameplay.
In the short term, though, as you place Pokemon(Trainers', Gyms', and Wild), write up three hypothetical players of your game - one who's an expert on all things Pokemon, one who plays casually but maybe hasn't played for a while or just isn't familiar with your fakemon(most of your players will fit into this category) and one player who has either never played a Pokemon game before or is too young to understand stuff like stat boosts. Generally speaking(but again, this is in the broadest strokes), I like the young player to have an estimated 30-40% chance of winning a battle, an average player having 50-60% chance to win, and an expert having 80-90%. Consider what parties these three players will have at the point you're trying to write encounters for, and build your encounters with those in mind(for instance, if you're in the early game with mostly normal, grass, water, and bug types, odds are that Steel types or Ghost types are going to be fairly difficult to defeat).
Gameplay "balance" (I use quotation marks because really you aren't balancing it so much as shaping its difficulty) is an infinitely deep rabbit hole, though, and I do recommend finding a dedicated playtester or two that can document their process for you, because while the method I described will put you off to a decent start, it's going to always require finer tweaks. And finally, this is just a simplification thing for me, I like to keep all levels as low as possible whenever I can - after all, Pokemon only go up to level 100(or whatever final level you decide on), so once you have that power creep escalate to its maximum, hopefully you're well into your postgame.