Are you making your game with RMXP? Do you have loads of rebattles with the same trainer, where the trainer might have the same Pokemon, but they'd just be a different level? Or do you want to be able to rebattle a trainer, and have their Pokemon become stronger or weaker without creating a new PBS entry? There is a very simple solution for you, and it goes like this. Go to PokemonUtilities script and anywhere (outside a def of course) place the following code:
The way you would use this is as follows: When creating a wild Pokemon Battle
Where type should either be "max" (to find the highest party level) or "avg" (to find the average party level). Quotation marks around the types are needed! The factor value has to be filled in if wanting to find the maximum level, whereby the max level would be multiplied by the factor percentage (where 0.01 is 1% and 1.00 is 100%)
If you'd like to place it within trainer battles, you'll have to go into PokemonTrainers, and replace
Where the switch X would be your trigger for level calculations. You can play around with the scripts to adjust it any way you want, but this is just a very basic method to return either the maximum level, or the average level.