My 5 Demonology Warlocks were awesome until they hit Ramparts.
Everything melted very quickly but they still couldn't do the last boss in Ramparts at L63.
I then switched to my 5xShammies and levelled them up to 70 with zero problems anywhere (except Sunken Temple).

I've recently revisited the Warlocks and have swapped out 1 Warlock for my (admittedly, already L70) Priest... with dedicated healing, they immediately cleared Ramparts and Blood Furnace with no problems.

So, I'd say, 1 Priest + 4 Warlocks