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.
Cows generally is best place. Best place to lvl nm merc is nm cows till hes lvl60-70, then you move do hell cows. You can kill all cows. Its faster that way and merc gets still good exp. TIme your merc kills cows and you kill all cows. You do 2 games that time. So you do all work.