The final boss of the Volcanic Fractal is a transformed version of the Champion Shaman you fought earlier. Rather than him walking, he is now flying around the central island, surrounded by lava. When you step onto the island, you and all party members will be effected by a debuff which remains for the entire duration of the encounter. The debuff causes burning if you standstill, meaning you and your entire party should continue to run around the shaman at all times, whether in melee or from range. You should also be aware that there are pools of lava that litter the island floor (which also cause Agony at higher levels), so be sure to avoid them as they also cause burning.
At intervals of 25% health, the shaman will once again surround himself with a shield and will begin to move towards more captives on the edges of the island. Once surrounded by the shield, the shaman will also summon Lava Elemental Grubs which die relatively quickly, that are incredibly useful for rallying off. With his shield activated you should quickly attack him to break it, with condition and haste skills proving incredibly effective here as the shield protects against individual strikes. The only attack from the shaman you should be aware of his aerial bombardment (what I would describe as a series of flame-type rockets) and his explosive aerial stomp (he remains stationary, flies up and then swoops down for huge damage). If you can avoid both of these, the encounter is little more than a tank-and-spank.
In the boss staging area, to your immediate left, you’ll see a row of humans waiting to be sacrificed and in between them, a skull altar. The moment you and your party arrive the shaman will become surrounded by an impenetrable stone shield. The only way to break the shield is to pick up volcanic stones that fall from the ceiling around the shaman. When the stones spawn they will also be surrounded by an area of effect spell that knocks you down. The most sensible tactic here is to have one member of your party concentrate on picking up the stones and throwing them at the shaman.
To make the encounter a little trickier, a series of veteran shaman will spawn at the back of the room (with the exception of one) who will slowly work their way towards the imprisoned humans, before pushing them into the lava. It’s in your groups best interests to protect all the captives as doing so will result in an extra reward chest. To keep the approaching shaman away from the prisoners you’ll have to use as many knockback, pull and cripple skills as you can. You should also be aware that at the furthermost side (towards the right) a single shaman will spawn next to a prisoner and instantly knock them into the lava. Placing a member of your party at the location to knock the Grawl back will give you a much better chance of obtaining the second chest (see screenshot below).