From Darkness, Comes Light
Desperate Prayer (Have I ever even used the other two?)
Slightly harder than Valiona and Theralion, but overall still rather trivial. The conventional textbook strategy is used to fight this boss. The encounter is consisted of 3 phases. Phase 1 consists of Feludius and Ignacious. Phase 1 ends when either one of them reaches 25% health (or was it 30%?). Phase 2 consists of Arion and Terrastra and this phase ends with either one of them reaches 25% health. Generally it is best to get both bosses down to 26% before pushing into the next phase instead of focusing and zerging one down to 25%. Phase 3 starts after all 4 council members merge together to form an Elemental Monstrosity and its health will be the sum of the remaining health of each council member.
Phase 1 -- Feludius and Ignacious:
Basically tank and spank. Try to avoid getting waterlogged from Feludius. Also when Feludius casts Glaciate move away from him since its damage decreases with distance. Ignacious will place a shield around himself and channel Rising Flames. Simply DPS through the shield and Interrupt him with Silence when that happens. If you become waterlogged, simply run through some fire patches on the ground, though that did not seem to work for me. Get both to 25% (or as close as possible) before pushing to next phase
Phase 2 -- Arion and Terrastra:
Again tank and spank. Grab the Searing Winds buff (run yourself into a mini tornado) in order to become immune to Terrastra's earthquake ability. Likewise, grab the Grounded debuff (run into a gravity patch) in order to become immune to Arion's Thundershock ability. Terrastra will cast earthquake first so get the Searing winds buff first. Again, get both to as close to 25% as possible before pushing into the next phase.
Phase 3 -- Elementium Monstrosity.
You will want the health of the monstrosity to be as low as possible, hence each council member's remaining health from the previous phases should be as close to 25% as possible. This is because there is a soft enrage mechanic in this phase. Aside from melee attacks, and dropping some icy puddles on the floor that shouldn't be stepped on, the Elementium Monstrosity also does a Chain Lightning ability that increases damage over time. Hence it is vital to finish the boss as quickly as possible. Rotate some healing CDs such as Desperate Prayer, and Vampiric Embrace near the end if necessary.