At the end of Pit of Saron, when you fight the Scourgelord, an unending onslaught of enemies attack the group of npcs who are supposed to keep them off you as you fight the boss. This is the set up for an easy rep grind.
All you need to do, is tank the boss off to the side, healer in a spot to cover him and the rest of the group, then have the dps aoe in the adds (You don't need to damage, just tag), and start reaping the benefits. The NPCs aoe down the adds, and you get 7-8 rep for every kill, and it starts addding up. Fast. Pictures below.
[Confirmation Needed] Additionally, as a warrior tank, I was spamming charge at scourgelord right as he jumped down, and as soon as he landed, he hopped back on to his frostwyrm and began flying around. This allowed for the whole group to focus on the rep grind.