killing the warlocks pet is a requirement for making the fight alot easier for you... personally when it comes to dealing with locks.. focus on the lock (put pressure on them).. have a few dot based attacks on the pet.. then once the pet has gotten down to say.. 10k health, swap to it and burn it down, then instantly swap to the lock and burn him fast and hope that the healer can't cope with the sudden 20-25% increase in damage the lock is taking..
if the lock manages to instant summon a new pet.. well try the same tactic again.. though you'll have to CC the healer a bit better as chances are they'll focus a bit more on the pet the 2nd time..
imo.. NEVER stop attacking the lock.. the moment you take the pressure off the lock they get the opportunity to start CCing freely and well.. everyone hates it when a lock CCs...
definately mana burn the healer.. but if they're good at pillar hoping, try mana burning the lock instead.. it'll force us to have to lifetap.. which is a dangerous tool to use in pvp..
so for the pointers..:
0. dispel all buffs from both the healer and warlock.. SPAM dispel at the start, it's such a benefit to you to do so
1. always have 1 guy on the lock.. keep the DK on him generally speaking
2. get dots on the locks pet.. then when its health gets to around 10k, swap to it and burst it down, but save your CDs so you can swap to the lock instantly and then burst him down (worst case, you force the healer and lock to blow their CDs.. best case you kill the lock)
3. mana burn the healer to hell
4. mana burn the lock if the healer is a dodgy git / if the lock is low on mana.. forcing him to lifetap is always good
5. if the warlock isn't affliction, dispel dispel dispel.. if the lock is.. be careful with dispels... you don't want to catch a UA
6. when you've killed the locks pet and you swap to the lock to nuke him down, after doing your instant burst (so SW:Death, penance..) spam dispel.. you want to remove any and all hots from the lock as well making sure to try and dispel his 'fel dominance' so he can't instant summon a new pet
7. ????
8. Profit