May be worth setting gained EXP to clamp at (Exp-To-Level /10)
This change will not make life more difficult for high level players, but will mean that it does not make sense for low level players to go to a high level dungeon when you get just as much XP from lower level ones with easier mobs.
At this time, if you go to the high level dungeon as a level 1, you level as follows for each 1 mob killed.
1-7
7-10
10-12
12-14
14-15
15-17
17-18
18-19
19
20
With a bow, a Ranger can do this in about 12 minutes, without a boost.
Clamping the Exp, will make it so that it still takes a minimum of 10 mobs for each level regardless of which dungeon/mobs you are killing.