How about, instead of constantly grinding the same enemies, your levels are based on the number of that monster you kill?
For example:
Kill monster A 30 times = 1 level up
Kill monster A 100 times = 1 level up
Kill monster A 200 times = final level up--
From here, you can no longer level up fighting this enemy.
Therefore, the cycle starts over--
Kill monster B (slightly stronger than A) 30 times =1 level up
And so on and so forth. Bosses count as an instant level up.
This way, you're inclined to find all the enemies in the game to reach full potential, and you're not stuck in the same area grinding for levels, taking longer to level up each time.