The lesson I learned on this fight is don't expect to see what you're shooting at. This guy is monstrous! The mechanics are very different than I remember in the past, but nonetheless fun.
Catch: Steering is hard! Add management.
This strategy is for the normal encounter. Heroic guide can be found here.
During phase 1, Rhyolith ignores players, and is steered by dpsing on his legs to turn him in a particular direction. You'll see a meter pop up on your screen that shows you what direction Rhyolith is turning. The more you DPS on the right foot, the more the meter will show toward the right, and the opposite for the left. Rhyolith will occasionally stomp the ground, and there's nothing you can do about it. If Rhyolith reaches the edge of his platform and gets into the magma, he'll drink it, and bad things happen.
So why are we steering this giant anyway? Well, Rhyolith has all kinds of armor on, and the only way we can remove it is to steer him over the randomly forming volcanoes. Lord Rhyolith's Obsidian Armor is reduced by 10% every time he steps on an active volcano. When Rhyolith reaches 25% health, he'll move into phase 2.
Sound too easy? It is, until you bring in the adds. Lord Rhyolith releases a jet of cinders, inflicting 15000 Fire damage to players within 0 yards and creates either 5 Fragments of Rhyolith or 1 Spark of Rhyolith. Range DPS' role on this fight is mostly to handle adds, while melee steers the giant around. Here's where it gets tricky. First off, you can't see very well because of the giant on your screen, so targeting becomes a challenge. Nameplates help a lot. (Just hit V on your keyboard and they'll show up if you haven't changed that bind) There are two types of adds. Fragment of Rhyolith – Fragments of Rhyolith have low health. If not slain within 30 sec, they inflict damage equal to their current health to a random player. In 25 person raids, they deal damage equal to half their current health to a random player, and Spark of Rhyolith – Sparks of Rhyolith inflicts 8075 to 8925 Fire damage to all players within 12 yards.
Priority on the adds is really to get them down as quickly as possible, but when faced with Fragments that are about to explode, make sure you swap to those and then keep burning the Sparks.
While all of this is going on, when Rhyolith steps on a volcano, he'll create a crater. When he's feeling saucy, Lord Rhyolith causes streams of lava to flow from a crater. The moving stream of lava inflicts 75000 Fire damage to any player in its path. After 10 sec. the stream of lava erupts, inflicting 150000 Fire damage to any player standing within a lava stream. So really, instant death.
The lesson on phase 1 is kill adds, and fire is bad. There's a lot of chaos, so it's important to have those priorities straight.
Phase 2: Burn the Armorless Giant
The positive to this phase is that the chaos stops with the adds and the volcanos. Your focus needs to immediately turn and DPS Rhyolith. Burn cooldowns, and light him up. Since he's armor-less, this phase is over very quickly.
What does a Hunter need to know?
Phase 1: Know what tank is handling which type of adds. Misdirect appropriately. The sparks will be tanked further off into the distance, where the fragments will likely be closer range. There is a lot of damage going out on this phase, so any mitigation will be helpful. If you're having difficulty staying alive, it might be worth glyphing Raptor Strike since you're likely to have a bunch of small adds close to use it on. Stay out of the lines, and staying vocal about when they're spawning will help your raid immensely.
Phase 2: Misdirect to the tank, since he no longer ignores his aggro table. Light up the boss. Blow cooldowns, use potions.
Hunter Loot from Lord Rhyolith
Arbalest of Erupting Fury (MM ideal)
Still having trouble? Check out Learn to Raid's video.