It depend what the problem here is. Does the exp given by each mob increase too slowly? If so then reduce the exp needed for the next level up. If it's because someone can "cheat" by killing bunch of wolf in an instant due to its high level and gain quickly exp,
then set a limit of level between someone and the target to obtain exp, something like under five you gain nothing. You can even set a price with that set, exceeding five level of you will give you more exp.
@RepresentingPride so far I am looking at the "no EXP for killing ants" thing and am struggling. Because the easiest way to stop MC level up is to ensure he can't access to the hunting fields and that is what the future plot entails.
Then have him get an object/skill that reduce the exp he get from killing or even prevent him to get exp from lower level than him. You can even use both.
And if you want an idea for a skill, just create a Pride passive skill