Duncan the Black, HM/NM

Quick question about Duncan the Black, first of all, Does killing Duncan in HM give credit for NM too? Or does it only count for one? Second question, can it be done with 7 heroes or do you need live people with you?

Also, can I kill him in HM, not turn in the quest, then go back in NM and rekill him? Or does he not spawn again?

Only the mode you did it in. If you want the master of the north title you'll need to do all the dungeons twice; once in each mode. Might be annoying; but you'll likely get several diamonds along the way.

Every single thing in PvE with no exceptions whatsoever can be completed with 1 person + 7 heroes in HM and without consumables. Some of them (like Domain of Anguish and the Underworld) are still very difficult but for the most part as long as you have half-decent builds you'll be capable of handling the content.

It's been a long time since I've done Slavers so I'm not entirely sure, but I'm pretty sure you need to do it twice.

Door to Duncan will be closed after you have killed him, regardless if you turn in quest or not.

Did this before the 7 hero update with 3 hero's and 4 henches and it wasn't that hard. If you don't want to use the method where u die beside him and let a nec hero kill him then flag back all your dmg hero's and keep your healers in range to heal you while you kill him alone since the dmg will only hit you. Just remember to put healers in first few slots so you can flag properly and swap out spirits.

FYI, for HM to count you actually only need to do Duncan's level in HM. The other bosses/levels can be done in NM, then switch to HM to take out Duncan for it to count in your HM Dungeon Master's guide (per notes on the wiki).