Equip him with Gaze/Tal's helm/Crown of Thieves, eth Pierre Tombale Couant and Gris armor or Black Hades with 3 IAS jewels and visit nightmare Baal minions. If he dies, resurrect him on the spot. At L75, just exchange the weapon with Reapers Toll and he is ready for hell mode. That's how I did it with nearly all my mercs.
Mercs get no XP from kills by your party members and their minions and he gets triple XP for his own kills, so it might make sense to level him without help from others.
from my experience, I have always found it easier to mainly level it (if there is a huge level gap between you and the merc) in worldstone keep of the difficulty closest to the suitable level; in your case nightmare worldstone keep. You should be able to get him to about 70 doing this and then take him to hell and let him melee/cast a bit there.
Just do a few worldstone keep runs and you should be fine.
sorry, since i recently got back to diablo 2 i dont have very good items yet. the only things ive managed to lay out for my merc are Bonehew socketed with ED jewels (given by a friend), Gladiators Bane and a life/mana steal helmet also like 40 mf i cant recall exact name. im going to do the NM baal runs with him, thanks for info
ok, here I come with something not mentioned. If you have a char able to deal damage to more than one unit at a time (basically any sorc, zon, ranged char, psnnecro etc), i used a psn necro, this is the place for you. Loads of semi-weak monsters, that arent too hard to kill, but there are TONS of them. The place is:
Flayer Jungle (hell!)
From my own experience, they die in one psnnova, which means they shoudlnt be that hard to take out with any other ranged/magic-using char.
I joined baalruns with 8players, and went right to the jungle. There is no need to teleport anywhere to find action, its everywhere. Besides, you dont have to worry about party members having the last hits, you are on your own.
Also, those small bastards barely hurt you
My own experience is that this works extremely well. My merc was levelled there from the lower 60's up to where he is now, at lvl **.
In an 8-player game (i join, and roam around until the other guys are finished with the baalrun and then find myself a new baalgame), the merc levelled from lvl 87 to ** in just 2 baalruns. Conclusion: Great exp, all the way.