Q:
Necro PVP & PVE Combos
Depends completely on your weapon choice, and what you mean by “Combos”. If you’re talking about optimal cast order to do the most damage it will typically rely most on using weapon swap effectively. Generally add vulnerability with Axe or Staff (3+4), then swapping to Dagger or Scepter to do damage either by straight DD (Dagger) or bleeds (Scepter).
As for your first combo above, you’d be better off using Staff 3, then 4, then 2. 3+4 gives AE vulnerability making 2 and 1 hit harder, so order should be Chilblains, Putrid Mark, Mark of Blood
For combo fields, Necros can set up Poison (Staff 3, Poison Cloud), Dark (Well of suffering, corruption, darkenss, power), Light (Well of Blood), Etherial (Spectral Wall)
For finishers, sadly we have only two, and both are staff. Staff 1 is projectile (sometimes), Staff 4 (when triggered) is blast. Using these with your combo fields effectively will help you and your group do more damage as well as survive.
Example: laying down spectral wall weilding daggers and running the mob over it keeps up stacks of vulnerability as well as protection on your self.