Yeah, quite a bit of the info in this thread is inaccurate. Enemy of one does not work in pvp. You do not get double damage from slayers in pvp. OP said this was a necro mage in wraith form (not tank, and I'm assuming not curse weapon).
The best suggest was to poison/mortal, but the poster didn't take the explanation far enough. The main reason to do this is, as that poster suggested, is that it will force them to heal with spirit speak instead of heal spells. Yes, they will heal less, but more importantly, spirit speak can be interrupted if you hit them, even if protection is activated. The challenge of course, is that a fully outfitted pvper will have potions & petals to deal with poison and apples to deal with mortal. So you'll either have to outlast them (most people don't carry more than 10 maybe 20 apples at a time) or find out what they aren't prepared to deal with. A bleed is another way to stop healing.
But really, if this is a necro-mage set up for defense (max DCI, maybe even parry), a dexxer is only going to be hitting them well less than 30% of the swings, which makes dealing enough damage to overcome their defense really tough. This is not, however, unique to this template. A good bushido archer healing with bandages, confidence & potions (easy with a balanced bow) is a terribly tough mark to bring down 1v1; and they're firing back at you to boot.
One thing that I'm not sure about is whether dismount will take them out of wraith form. I know that dismount will knock a ninja out of animal form but I think it may not work with necro forms. Truth is, most people you find in wraith form are dying to get out of the form in PVP because it requires them to be on foot. As a mounted dexxer, this should at least give you the advantage of fighting or getting away.
As is the case with most pvp, simply trying to smack someone down with more damage is pretty futile. To overcome a good pvper you almost always need a combination of things - different spells, poison, mortal, bleed, dismount & damage. Just focusing on damage pretty much never gets the job done unless it's a gank. Trying to figure out an opponents weakness is an interesting aspect of pvp, and it's the reason why most dexxers & archers carry 3-4 weapons (something with disarm, armor ignore, bleed, poison, dismount, etc.) Too bad most pvp encounters are pure ganks.