keep in mind your merc gains for exp when it kills rather than you killing. what build are you using with your merc?
In my instances, i had a wind druid lvl 80 with an nm act 2 lvl 45 might merc...I kept on doing trav, shenk, eld, countess or any pubby runs. by doing my merc was equal to my lvl (90) and found great items.