Possesed weapons are really not all that good. Most of the times you can buy better ones from the merchants. Depends what your total crit chance is and how far you are in the game. If having a few more points of strength does not add a lot to what you have now, then go for crit chance. Having high crit chance destroys everything. But if it brings you at like 15% crit chance then don't bother. Lifesteal could be the first choice, but yet again it depends on how much life it steals, and your total hp. If the health regeneration is barely noticeable, then there not much point to it. I was lucky and stumbled upon scythes with extremely high lifesteal. Nightmare was a walk in the park on my second playthrough.
About your other post, it's been a while since i played but on my second playthrough on nightmare, I found myself using almoost exclusively the harvest tree. And that other one that raises crit chance and damage. I dont' remember what it is, the tree on the bottom left side. Max both the harvest tree and that one, and you're good to go. Necromancer path is pretty useless I don't use it. Maybe just the armor but then again it wastes points that you could use on those other abilities i mentioned. About the moves, well I use the three main ones for the scythes, something like x wait x, xx wait x, xxx wait x or something like that, but don't get any of the ones for the secondary weapons or the counters they are almost impossible to do. I don't use secondary weapons at all.