Hi,
It seems that maximum experience (after 4th quest = for 801 lvl) the maximum experience you get from monster is 7 624 155. I have tested this in arena / ferea / cubera mine / deep dungeon and also Nixie. Ferea gives lower exp per mob = 5 467 608.
This is a bug that should be fixed. Going from 800 to 801 in arena (mobs with lowest HP = fastest clear) will take approximately 4 hours. This means that 4th quest ML is not possible to do..
Thank you
Regards
Shmav