Ok, understood now. I just think highest level players will farm the lowest players that will try to create a Challenge. They won't have many chances. Maybe you can set a countdown time for each pokemon. Exemple: If I used articuno, I just can use him again in the next hour. So, stronger pokemons can't be always used, forcing the player to use many different pokemons.
You do make a good point about higher level players farming lower level players with early starting Pokemon. I will try to think of something so this doesn't happen. At this point in time I wanted to get the battle system out as I promised as I did say it would be released either yesterday or today.
Maybe I can add Battle tiers based on player levels.