and play your merc in areas closest to his level and you'll level him quickly. If you do something like take him to the cow level, but he's way too low, he won't get much exp for the trouble.
I just leveled a normal merc to lvl 78.The pain was in the beginning.I played dozens of boring normal Tomb runs,Travi runs,act4 runs,Bloody runs,etc.The easiest was when he was around 45.Then he was gaining 2 levels in a 8 player cow game but I was doing most of the kills(remember that mercs don't get exp from party kills)