Core necromancer can do everything. It is an incredibly good teamfighter, bringing strong controls, sustain while still having good damage. If needed it’ll also hold duels very well, with the ability to win if your opponent isn’t careful.
Out of Death Shroud your main purpose is gonna be to slow down enemies and generate life force that’s required to have access to Death Shroud . To do so you want to start every fight on staff, harass your enemies from long range, every staff skill will generate life force on top of being really annoying for enemies.
Out of those you have a few way to pressure, Death Shroud skills are the best. Dark Path (careful not to use the 2nd part of the skill if you’re trying to damage from range as it will port you on the person you hit) and Tainted Shackles can be used off cooldown. Dhuumfire will put a lot of pressure but be careful not to abuse it as you’ll lose life force and Death Shroud is also one of your best way to stay alive if you’re focused, as it offers an extra health bar with damage reduction.
When you swap back to staff, Putrid Mark deals a lot of damage as well as transferring conditions. Leaving Reaper's Shroud will also cleanse 2 conditions from you Spectral Walk is your best kiting option as it stunbreaks and lets you port back to the location you first used it at (up to 10 seconds). Necrotic Traversal is also a stunbreak and will port you where you set up Summon Flesh Wurm. You can use it as a basic panic button but be careful to not isolate yourself from your team trying to survive, especially if facing Herald.