I've posted a few army lists now. Whenever I have a necromancer on a nightmare I'm told to put it in a group of skellies. It seems to me the Necromancer would be in grave danger with this strategy. I know about the Necro Bunker but is there another strategy I'm missing here. The only thing I can think of is that ther necromancer leaves the unit before the charge but that seems might dangerous.

In a unit of skeletons, it can't be picked out - despite being bigger, it's not bigger enough. I personally wouldn't, admittadly, though, as I don't really like the look of it.

In terms of safety though, it's fine - you don't _have_ to charge with said unit, and the necromancer will probably be fine unless he's charging chosen chaos warriors or something,

Well a necrobunker doesn't consist of skellies per sé, it could be zombies :P
I must admit, since 7th ed I haven't run a list without a necrobunker - minimal unit of zombies allowed. And I have lost my necromancers only once ever since. So, not bad at all I'ld reckon.
I don't quite cope your fear of this strategy. As a necrobunker is always placed just behind your main battle line. So one has to plough through this line first or has to out flank you completely to get to your necromancers.
I'm not saying that this is thé best strategy ever, but it is quite a save and proven one.

Yours

if you have a necro on foot then the best place for him to be is in a unit o skellies or zombies..Now whats this with a necro on a nightmare in a unit of skellies?

He´s the bigger target in that unit and therefore he can be shot at..Not recommending that but if you field him in a unit of black knights ..thats something different=)