So lets start with the guide :)
Level 85+: Take the quest at the beginning of the instance from Lin Yan, you can get it every time you enter. You need to finish this quest to get the quest item 'Faithfulness'.
You have to kill the Glacial Shades to open a gate, if you go through all opened doors you will do all in the right order. Bosses open doors too, so there is no skipping bosses in here.
2. Thunderclaw: This boss summons traps, typically 3 spot where players are at. You have to run around a lot. Herc can tank, but be sure to call him back every time a trap forms around him. Don't stay in the traps, they deal massive damage on you, barb can remain in one spot to tank. Move away once one forms around you.
3. First talk to Holan, he will give you a quest. Kill as many elementos as you can (seriouslly, kill all if you can). You have 30 seconds to kill each one of them before a new one respawn with a full hp, be sure to focus fire one at a time. After 30 seconds the mobs disappear. If all are gone the boss Diabolic Shocktrooper wakes. He random seals like the genie skill Occult Ice and sleeps. When you have the seal bubble around you, you get some bad damage. Clerics need to purify. After you kill him two Frost Bishops spawn, with foxes around them. These are easy enough to avoid drawing aggro from; alternatively 1 person grab aggro, cleric team heals, rest of the team focus on getting the Frost Bishops killed asap. The Frost Bishops AoE interrupt and damage.
4. Level 85+: In a small room the NPC Shiner Nin appears, all talk to her, kill Cyclotra Styren and Glacix Dragoon that spawn and report. The dragon AoE stuns. It respawns a while after it's killed. Cyclotra Styren does AoE. This quest will give you the quest item 'Faithfulness'.
5. Dreadindra: He calls up Frost Bishops, random aggros and does purge debuff then. Barb/herc tank boss try to keep him on you, rest kill Frost Bishops as soon they appear and DD on boss in between. Cleric best team heal. Don't worry to much about the debuff, the bishops are worse.
6. Asoteric Runewolf: This boss shouts, keep a close eye on that. When he says SLASH you better be far away, because he does a AoE that 1-hits all. After the slash run back to continue killing. He slows with his attack. It is wise for the venomancer to use soul degeneration so that the boss will not regenerate his HP while the squad is running.
7. Messenger of Fear: Does AoE sleep and damage. Cleric team heal.
Level 85+: After you killed this boss the NPC Yi Lam appears. Talk to him and he will teleport you near next boss. If you are below level 85 you need to walk.
8. Oceania Master: Stands in a big room filled with mobs. Kill all mobs before hitting the boss. Let the barb or herc tank the boss. Do not use AoE. Don't hit the minion that spawns next to the boss, telling you to try hit him. Ignore this minion, don't AoE it too... If you hit the minion he does a severe AoE killing most people in your squad. Kill all hands asap, before they clap cause they kill almost everyone if they clap. Best put archer and/or BM on killing hands.
Level 85+: After you kill this boss the NPC Celestial Sister appears, talk to her. Hit as many of the Frozen Heads you can, they give EXP, and afterwards the NPC gives you a second quest item 'Sincerity'.
9. Decaying Fragrance: This one does the axe debuff like one of the last bosses in FB 69. Barb has to tank it (or Blademaster). AoE stuns.
10. Asotheric Runephoenix: Shoot him, run to lower him so all can hit him. He AoE seals and does damage, drains life. Just DD, cleric team heals. It is wise for the Venomancer to use soul degeneration when amplify damage is on cooldown because this boss has a great HP regeneration rate. After the phoenix dies, you need to destroy the remains Blazing Phoenix Soul too.
11. Holeen: Lure the Holeen Body. After he's dead Holeen Champion of Fientazzar spawns. The big one random aggros, seals, AoEs and buffs himself at around 30% of his HP. Cleric team heals (usually: Regeneration Aura) and the venomancer should purge.
The last 3 bosses are optional so you can choose if you don't want to kill them or if you want tell the squad that you go together and kill the last 3 bosses.
The Last 3 bosses are only good for more exp and again you don't have to if you don't want to...
Here is the map of FC:
