Tank him far enough from the fountains in-zone to keep his stoneskin off him but within range of a raid force that's stacked up close to, or in, the fountain.
Every 45 seconds he will shout "Catch!" and up to 3 raid members will end up with a dull elemental. These players must jump into a fountain to put themselves out. While on fire they tick damage to themselves as well as those around them (including other people who are on fire!) If anyone dies with the elemental on them, its a raid wipe.
Warning to watch for elemental:
<Trigger R="Catch!" SD="Fountain" ST="3" CR="F" C="AoM" T="F" TN="" Ta="F" />
Debuff placed on the raid at the start of combat called "Punishing Impatience" comes in as a steady tick based on the damage dealt to the named. No big burns, especially if you've got low resist values. Essentially you can only burn as hard as your healers can keep you up.
Every 20% (80%/60%/40%/20%) the named will require a scout to grab agg and tank it. Any tank who tries to hold it will die. Damage will be reduced for the scout, so nothing special really needed to keep them up. Green text afterwards lets the tanks know when to snap it back.
Trigger for scout to snap:
<Trigger R="Your fighters are nothing to me, their head crushes so easily under my boot!" SD="Scout snap" ST="3" CR="F" C="AoM" T="F" TN="" Ta="F" />
A word of caution; Using Verdict while under the effects of the dull elemental will sometimes lead to a kill AND a raid wipe. Damage from Verdict is returned via Punishing, killing those on fire, wiping the raid. Its happened to us twice but always got loot. Better safe than sorry, though.