There are mercenaries suppressed to help with leveling, when there are no one to travel with. When you kill a mob, you get +/- 100% exp. But when your mercenary lands a killing blow, you get about 50%, even if they are supposed to HELP you with leveling up. As a result, they do the opposite sometimes. Think also could be fixed and help newbies to level up faster.

As a side note, maybe it's a good idea to add 30-35 lvl mercenaries to compensate the low playerbase, though I guess it's for the gameplay forum.

Those mercs should now properly give you good XP even when they get the killing blow.