With a necromancer in PvP there are skills you WILL need for fighting. These skills are Bone Prison, Iron Maiden, Bone Spirit, Fire Golem, and I like to have Decryptify. On below I will list how to fight each character and these will be important to read. Oh and one more thing try to use Fastest Run/Walk boots and Blink Bats form if possible
Below gives tips on what to depending on what class the enemy is:
Barbarian
I learned this tactic from a Level 80 Necromancer. You will need to have decent running speed for this to work. First make a fire golem and run, let the barbarian chase you. Iron Maiden him when you have a chance. When he goes to WhirlWind, you just run of to the side and cast bone prisons on him.
Since the bone prisons have life he does damage to something which sets the iron maiden off to kill the barbarian. You may need to do this couple of times and remember if he leap attacks you, just run and it'll miss.
Paladin
Now this is a easy tactic to do. There is two ways to perform this one. When he is charging you, you can either use bone prison or decryptify to slow or stop him. Then cast bone spirit till he dies. Also to make sure they don't get out of bone prison, just iron maiden them and they will know its over. (My Level 54 necromancer kills paladins who are on the ladder)
Amazon
Amazons are the hardest to kill once they master there ability to slow missiles. But don't worry, there is a way for the Necromancer to kill all. I hope you have a fast cast rate, because you will need it here. Try to get a good distance from the amazon than cast good amounts of Bone Spirits. then run up to her and Bone Prison her about four times. Since she can't move. all the Bone Spirits will come and kill her :).
Sorceress
This trick I only seen done once (agents my sorceress). Put Bone Walls all around your self. This is cool because Bone Spirit travels through Bone Walls but Frozen orb does not. Just simply wait for the sorceress to come close and then the Bone Spirits will kill her.
Other Necromancers
Now I always win this one. This is basically the paladin strategy but it' a little harder because they have Bone Spirit too. Just get them on the run with Bone Spirits chasing them and try to get close for the kill. Bone Prison is nice here but decriptify is real good. Try to use a combo of each and when they are stuck Bone Prison, all your Bone Spirits will hit him.