Reaper of Souls - Achievement Guide - The Guardian

Have all of the conversations with the spirit of the Barbarian's special character in the Pandemonium Fortress. Begin at Act 5.8 (Angel of Death), waypoint to the Pandemonium Fortress, speak with Tyrael to advance the quest, and simply continue on.

On Pandemonium Fortress Level 1:
0:00 - Recognition - automatic, simply walk down the stairs.
0:19 - Angel of Death - automatic, simply approach The Guardian, though you can speak with The Guardian to have the conversation again.
1:07 - The Guardian - speak with The Guardian.
1:36 - Spirits - speak with The Guardian.
2:11 - Defeating Malthael - speak with The Guardian.

Make your way to Pandemonium Fortress Level 2, defeat Seraziel, break the Soul Prison Chain, and you can then have the remaining conversations:
2:40 - Vigil's End - automatically starts upon breaking the Soul Prison Chain, though you can speak with The Guardian to have the conversation again.
3:13 - Final Rest - speak with The Guardian.
3:48 - Death - speak with The Guardian.