Check out act 3 - choose Siegebreaker quest, go to last wp ( so you will have Tyrael with you ) and then go back to town and through Arreat Gate to "The Battlefields". It's a small area, but often with goblins + at least 2 champion packs and some random quests. But thats not the best part. You have like 25% chance that there will spawn big tower with gold chest at the top or with bunch of soldiers . But dont open gold chest at start, because moloks will fly to you, including champion pack + its better to open it with some nephalems buffs.
Either way ( if its gold chest or soldiers ) its a great place to easly kill champion packs if you have char with ranged skills. You simply need to pull all hard mobs from The Battlefields to the spot at the bottom of that big tower ( but not from the side where the stairs are, because they will just climb on them ), than you can just die, respawn and go to the top of the tower and kill them from the top, being totally safe ( well, not totally, once a champion pack tele'd on me from the bottom, but that was just once + the only skill that can reach you are frozen orbs, thats it ).
So after cleaning Battlefields you have between 2-4 nephalems, so you dont need much to reach 5 nephalems ( you can f.e. go straight from Battlefields to the bridge, there is always one champion pack there and you can also pull them to the Tower ;p ) and when you have it, go and kill Siegebreaker, he is very easy and you have 100% for 2 rare items ( at least till 1.0.3 patch ).
Dunno how long takes one run, but its quite entertaining for some time.
Of course farm there only with that big Tower, its much easier and you have like 50% chance that there will be gold chest at the top of it.