Warlocks are the toughest caster class to fight because of their stamina (good warlocks have almost as much health as warriors in addition to life tap), their shadow resistance, and their fears.  Keep the pet feared and wear the lock down with Flays and your DOTS while healing yourself.  It is imperative to dispel the lock several times and then dispel any debuffs they put on you. The will cast “Curse of Tongues” on you most likely, so try to stick to instant spells.  If you get a chance, drop Shadow Protection on yourself (though it is expensive you will significantly decrease your damage taken).  Their pets are relatively irrelevant if they are out, just keep them feared. Fighting a lock is a battle of efficiency.  Finally, they might try their hand at mana burning you.

Felhunter locks are the absolute worst nightmare for Priests.  By having a felhunter, the warlock has tremendous resistence to your spells, and it is essential that you concentrate on surviving and mana burning the lock. Leave your Shadow Word: Pain on the lock and keep yourself alive, while mana burning him (unless you receive the curse of toungues).  Lock/Priest fights are very often reduced to stick beatings.  If you happen to stumble across the felhunter, immediately concentrate on the pet with Smite.  It has very low health, and his death will take down the lock.

Warlocks are the hardest class for a priest to fight.  Holy priests have a better chance than Shadow priests on surviving. However, neither has a very good opportunity.