1. Choose Act I
2. Select Explore Cellar
3. Teleport to The Old Ruins
4. Skip all the mobs, run right to the cellar
5. Kill the captain
6. Repeat.
Note that this is tougher to solo on T6 without a group due to him being able to freeze. I've been farming this solo on T3 with a WD at 193k DPS, it takes about 1.5 minutes and grants 3.6 mil XP and 30k gold on T3. Higher torment will grant more XP/gold.