Relevant Talents used:

Void Tendrils
Mindbender
Desperate Prayer
Divine Insight (Twist of Fate works just as well)
Divine Star

iLvl: 505

This fight is divided into 2 phases. Phase 2 is a tank and spank so I will only talk about phase 1 of the fight. To trigger phase 2, the legs must reach 25% health (they have a shared health pool)

Phase 1:
Phase 1 is all about removing obsidian armour from the boss. The boss starts with 80 stacks of armor and each time he steps on an active volcano it will reduce his armour by 16 stacks. The boss does not attack during phase 1 and can only walk forward. To steer the boss, damage must be done to his legs (ie. attack his left leg to turn left).

Volcanoes:
The boss will periodically use Concussive Stomp which spawn 3 volcanoes. But he will only activate one of them. If the boss steps on the active volcano he will lose 16 stacks of armour. If he steps on any other volcano it will cause magma flow, which can be avoided. As long as the active volcano remains active, it will cause periodic fire damage, as well as applying a stacking debuff that increases fire damage taken. This debuff should be minimized to prevent excessive damage intake. Therefore, drive the boss onto the volcano as soon as it becomes active!

Fragments/Spark of Rhyolith:
The boss will periodically summon either 5 Fragments of Rhyolith or 1 Spark of Rhyolith. Fragments can be aoe'd down within 30 seconds or they will each deal their remaining health as damage. The Spark of Rhyolith must also be burned down quick since for each 5 seconds it remains alive it will deal 10% increased damage.

Drinking from the magma pools:
The boss will cause raid wide fire aoe damage whenever he reaches the edge of the platform. He will then do a U-turn and continue on his way. This aoe damage varies depending on the amount of debuff stacks generated by any active volcanoes. Without any stacks this damage is quite tolerable even without cooldowns, and sometimes it may be beneficial to use this strategy to cause the boss to U-turn since not only it's faster than continuously attacking his leg, but also it allows more time to focus on the adds (fragments and sparks).

Concluding remarks:
Phase 1 does require some good RNG, especially with the volcano spawn positions as well as the position of the active volcano. If boss continuously activate volcanoes that is directly behind him, then it is very likely for the active volcano to remain active for a long time, causing debuff stacks to reach unhealable levels (it's still healable with Flash Heals but Flash Heal spams means falling behind on adds and boss steering). Cooldown usage isn't as crucial, but dispersion should be saved when boss is drinking from magma pool while high debuff stacks are present, and VE should be used while killing sparks since its the main source of single target damage that's not mitigated.