I'm trying to beat the Elite Four on PRO, but their Pokemon levels are "??", so I don't know what level to train my Pokemon to.
The levels of the Elite Four and Gym Leaders are unknown on purpose. The levels are hidden to make the game harder, so you do not know what levels you need to grind to to beat a certain gym.
The level formula of the gym leaders' Pokemon should be something like this:
(Gym Number x 10) + 5
So according to this Misty's pokemon would be level (2*10)+5= 25.
Elite Four would be around level 85-90.