The Best Strategy Guide Ever

This is Sartharian of the Obsidian Sanctum (picture swiped from Wowwiki):

This is a fairly typical diagram from a raid strategy page (also from Wowwiki), showing how the main tank (green dot) is to position Sartharion (yellow ellipse) and where the meleers (purple dots) and ranged damage-dealers and healers (blue dots) should stand when lava waves (orange bars) come in from the left:

With that in mind...

Go look at this guide to the Sartharion fight.