While Beth’tilac did have a mechanic that forced you to split up between the group upstairs and the group downstairs, both parts of the raid essentially handled a tank and spank, with or without adds switching to a tank switching single boss mechanic in phase 2.
Lord Rhyolith is far off from this, the first phase being (easily) the more demanding of the two. Depending on whether you’re dealing with the fight in the 10 or 25 man version, you’ll be working with one or two tanks for the fight.
During the first phase, the only real tanking job is to handle the various adds that will spawn, either the small group that need to be AoE’d down as quickly as possible or the larger add that pulses a Hellfire centered on his position.
With the smaller adds, since they need to be burned down ASAP, it would be a good idea to try and align your short term AoE cooldowns (like consecrate, death and decay, and thunderclap) to line up with their spawn timers as much as possible. Because these smaller adds are meant to be killed quickly, it would do your raid a whole lot of good if you had those abilities ready the second they spawned so that they could just drop that AoE damage right off the bat.
The larger add, as mentioned, is tanked away from the group and pulses a Hellfire effect around itself. The Hellfire doesn’t really do all that much on it’s own but with the stacking debuff from the volcanoes on the field that increases fire damage taken, that Hellfire can cause a huge amount of damage if enough people are in range to be hit by it so do your best to move it around in a way that prevents a lot of people from getting hit by it. It can help a little if you designate 4 cardinal points on the island using the proximity raid markers so that you can call if you’re moving in a given direction, allowing your raid members to recognize more easily if they’re in the way or not and whether or not they need to step quickly.
Phase 2 begins once his feet drop to 25% HP. It’ll start with a raid wide AoE pulse leading into a simple tank and spank burn phase. Watch your health at this point, don’t be afraid to use or call out for cooldowns to help keep yourself alive and if you’re two-tanking it, keep alert in case your first tank drops and you need to pick it up to save the day.
During phase 1, if you have an extra second here or there, try your best to keep whatever debuffs you have on the legs. Things that reduce attack power or attack speed won’t matter too much but the other debuffs that you may have that reduce armor and the like may be invaluable depending on who you have attacking the legs.
The first phase is the hardest part of the fight so once you have that down, know that the end is in sight and keep on trucking.