Nightmare Level for Necro

[Mordeeth]

What level should I start doing NM?

Right now I am level 41 and doing Baal runs.

Also everyone says that I need to have a Act 2 NM Merc (Might), but I am somewhat confused on what level he will be when I hire him...and what level I will as well...

[Nimbostratus]

I usually go on to Nightmare at level 40-45, but you can safely go on a little earlier too. For Hell, 70-75 is normal.

Mercenary levels are always within a few of your own (under usual circumstances, at least).