Whats about leveling for a multiplier of your current team
average gain (first fight util now)?
When you have 2 Pokemon in your team, lvl 10, 30 the average is 20.
You are fighting the Trainer with normal level.
then ur leveling to 20, 40. You have an average of 30 then, gained 10.
So the Pokemon from the enemy gains 5 level (with the multiplier of 1/2)(maybe something like random -> gains 4-6 level, or with a multiplier depending on the level (low level a bit higher than high level))
I would say this:
Difficulty 0: 1/2 multiplier
Difficulty 1: 3/4 multiplier
Difficulty 2: 1 multiplier