The second (and last) boss of the Throne is allegedly one of the hardest. I've only personally been in BRDescent but the first two bosses seem to be a gearcheck (Magmaw) and a skillcheck (Tron Council).
Magmaw puts out a lot of raid damage and focused tank damage, and spawns adds that need to be range AoE'd. If the tanks get a timer that shows when Mangle hits (~50K hit, then chewing for 20K/tick) and make sure to have a solid cooldown up for it the fight becomes a lot easier because he can hit with Mangle, a melee swing and a fireball at the same time for 110K+ damage. Other than that the dps kills larva adds, doesn't stand in pillars of fire, and two guys jump on his head when he falls (happens just after Mangle) to spike it for a high damage phase (in addition to stopping the tank-chewing). High damage but a straightforward fight.
The Tron council is four dwarf golems each with a different ability. Two will be up at a time staggered (so it starts with one, halfway through his energy bar a second activates, when it runs out it deactivates and a third activates and so on). Each one has three different abilities, usually a single attack, an area attack and a shield. Every ability is on a set rotation, but it can get chaotic to watch those. Important to note is that shields only go up below 50 energy, so there will always be a boss that can be hit.
--Arcanotron: Arcane Blast at a random target, can be interrupted. Drops pools of power that boost the damage of anyone in them (pull the boss out, ranged stand in). His shield boosts his damage and haste every time he's hit (buff can be spellstolen, Arcane Blst still interruptable through the shield).
--Toxitron: Spawn slimes that pick a random target and go for them, exploding if they reach (Focused guys run away, everyone else kills the adds). Create clouds that boost damage taken (players get out, if you can get the boss in there it's a bonus but make sure the tank doesn't get hit). Shield stacks a dot to people who hit it but also boosts their damage (don't hit it).
--Magmatron: Targets one player and then nukes their location (target stands still, everyone else gets away, healers heal the guy). Floods the room with fire (mild damage unless boosted by Arcanotron's ability, AoE heal). Shield explodes for massive damage if he's hit too much (won't break on one hit, but don't hit the guy).
--Electron: Targets one guy and gives him a damaging AoE (get away from other players). Chain lightning (spread out). Shield does AoE when hit (don't hit it. Notice a pattern?).
It's random the order bosses cycle in, so you may have Arcanotron spawn right after Magmatron, or you may never have to deal with both of them up at once. The next golem to spawn is clearly marked, so tank-swapping shouldn't be an issue. The biggest ones to deal with are Arcanotron's interrupts/buffing pools and Toxitron's adds, those can easily cause wipes if not taken care of.