I know this is already out there to a degree, but when anyone ever mentioned it they never told me how far you can take this. i.e. Farming aspect ghosts or helping you kill some for that quest was all I knew.
Start the game in either A3 or A4 at the quest where Tyrael joins you. That would be Seigebreaker (if you select the sub quest to Kill Seigebreaker, it will give you the waypoint just before his portal) in A3 and Prime Evil in A4. Go to the waypoint for the quest to activate Tyrael as your follower.
Now you can go ANYWHERE in the Act before that and use Tyrael to help you farm. This is especially powerful in Act 3 dungeons due to the tight spaces and corners. Between Tyrael and your follower, you shouldn't have any trouble with any elites, as Tyrael is so powerful. Die? Who cares. He'll do some clean up for you if you stay dead.
Max your Neph Valor using Tyrael and go kill Seigebreaker. I find this to be easier AND faster than farming the Butcher. Not to mention you get easy A3 drops.
Sorry if this is old news. I just noticed people mentioning bits and pieces of it without going into the full potential.