As a shadow Priest, the key to winning in world of warcraft PVP is to think like a tank instead of like a spell-caster. This is a difficult change of approach, especially for players who are used to playing their spell-caster as a "squishy" class and who prefer to avoid close range combat.
   Shadow Priests are different class and require a significant change in strategies in order to be successful. Like Mages, Shamans and Druids, they lack the enormous crits, high damage spells and mana efficiency. What is more, they do not have pets or a large variety of spells like Warlocks.
   Without doubt, shadow Priests are the toughest spell-casting class for melee attackers to defeat. They have the highest physical damage mitigation of any spell-casting class.
   It is simple to use this class feature in combat against melee classes. In fact, do not move at all except to change directions for mind flay. As a shadow Priest, you are gambling that shadow word: pain, devouring plague, mind flay and vampiric touch will deplete your opponent's health before they can break through your armor, armor bonuses and shields and outrun vampiric touch with their DPS. A well-timed psychic scream, especially just before a mortal strike, eviscerate or windfury will often close the door on a shadow Priest loss.
   The shadow word is one of the strength of shadow Priest. Shadow word: pain has the highest per tick base damage and therefore benefits the most from shadow bonus damage. Unlike Warlock damage over time spells, shadow word: pain can also cause a stun through the blackout talent, and in fact, rank one of this spell is often used as a ranged stun against running opponents such as a flag carrier in the Warsong Gulch PVP Battleground.
   The ability to heal is another for shadow Priest in PVP. Indeed vampiric touch is a nice ability, many shadow Priest players overlook the importance of dropping shadowform when necessary. Shadowform is primarily offensive class ability, should be viewed as optional from the standpoint of a PVP battle where health is becoming an issue, especially against a class with significant burst damage capability. Formidable is the shadow Priest who lands a greater heal mid-battle against a class that cannot heal at all, especially if it crits.
   Constant attack is the general philosophy of the shadow Priest. They don't avoid, run and concern themselves with numbers of opponents. Compared to other spell-casters like Warlocks, Mages, Druids and other Priests, shadow Priests are like blocks of solid iron to a melee opponent and very confusing to other spell-casters.
   Mind flay is one of the best openings for a shadow Priest. As strange as it may sound, landing a mind flay at maximum range gives a shadow priest a variety of options. If the opponent chooses to close, they are walking right into a psychic scream and the loss of their trinket. If they choose to run, the shadow priest can simply allow shadow word: pain spells to tick while they try to stay in mind flay range. If the opponent answers with ranged attacks themselves, the shadow Priest can simply answer with vampiric touch and give their opponent a large damage penalty while winding up a mind blast.
   Keep in mind that the shadow word is the important key while fighting in shadowform. Mind flay is far more versatile and far less expensive mana-wise, though it does less damage. Shadow Priests are notoriously mana-inefficient, and so must do what they can to preserve as much mana as possible by employing such strategies as downranking psychic scream and possibly putting a point in vampiric touch at higher levels.
   Shadow Priests have always been an extraordinarily powerful class and build for WOW PVP. The class is very simple and somewhat limited by a "come as you are" itemization system. The lack of efficient mana regeneration is true weakness. However there is no class or build in World of Warcraft that can count a shadow Priest as an automatic win with full mana.