What is the best tactic for killing all bosses in final of The Secret Library Quest?

Question

In final of The Secret Library Quest, a team of ten players have to face six bosses:

The Spellstealer , 

The Scion of Havoc , 

Brother Chill i Brother Freeze  , 

The Devourer of Secrets , 

The Scourge of Oblivion .

I'm curious what is the best tactic to defeat all bosses.

Answer 1

• Final Battle

After killing all 4 previous bosses, you can enter the teleport to the final battle. From the entrance, go south and then follow west. Go down a few floors until you reach a small room here. 

Enter the teleport to find the room with an  Ostentatious Bookstand. Unlike the other boss fights, this once can be fought with a team of up to 10 players.

You'll be teleported to a large room.

With NPC  Cerebrir and Magic Discharges. After 30 seconds, several creatures will start to spawn in the room:  Rift Spawns,  Rift Minions,  Imp Intruders,  Ravenous Beyondlings and  Yalahari Despoilers. One minute after this, the following message will appear at the center of the room:

THE NORTH EASTERN WING HAS BEEN BREACHED! RUSH TO CRUSH THE INTRUDERS!

•  The Spellstealer

Enter the teleport to the northeast. Inside, you'll find  The Spellstealer and  Demon Slaves. The Spellstealer is initially invulnerable to all attacks. To kill it, it must be lured to the vortex corresponding to its color: if the boss is green, lure it northwest to the Creation Vortex (The creation vortex consumes the stolen energy!; if it's red, lure it southeast to the Destruction Vortex (The destruction vortex consumes the stolen energy!). When the boss steps on the correct vortex, it changes to a light gray color and becomes vulnerable. After some time being hit, it will become invulnerable again (The spellstealer absorbs the power of destruction! or The spellstealer absorbs the power of creation!), and it will have to be lured to the other vortex. Repeat this until it's killed and then go back to the central chamber where a new raid will start (After the death of the spellstealer the invasion of the central chamber continues!) with the addition of  Invading Demon‎s.

•  The Scion of Havoc

After another minute killing the raid, the southeastern wing will be opened:

THE SOUTH EASTERN WING HAS BEEN BREACHED! RUSH TO CRUSH THE INTRUDERS!

Inside this wing you'll find  The Scion of Havoc and several  Spawns of Havoc. You should avoid killing the  Spawns of Havoc, since when they die they will explode and deal 1000+ damage to all players around it. This damage also seems to heal The Scion of Havoc. Thus, in order to kill it it's better to trap it in a corner while one or two members of the team keep the  Spawns of Havoc away from the boss. After killing The Scion of Havoc, go back to the central chamber where a new raid will start (After the death of the scion of havoc the invasion of the central chamber continues!).

•  Brother Chill &  Brother Freeze

After another minute killing the raid, the southwestern wing will be opened:

THE SOUTH WESTERN WING HAS BEEN BREACHED! RUSH TO CRUSH THE INTRUDERS!

Inside this wing you'll face  Brother Chill and  Brother Freeze, as well as  Biting Colds. The Brothers and the Biting Colds heal each other, thus, in order to defeat them you have to keep them away from each other. After killing Brother Chill and Brother Freeze, go back to the central chamber where a new raid will start (After the death of the frozen brothers the invasion of the central chamber continues!).

•  The Devourer of Secrets

After another minute killing the raid, the northwestern wing will be opened:

THE NORTH WESTERN WING HAS BEEN BREACHED! RUSH TO CRUSH THE INTRUDERS!

Inside this wing you'll face  The Devourer of Secrets together with  War Servants and 4  The Book of Secrets. Do not destroy the Books, as they will make  The Devourer of Secrets stronger. Simply killing the boss while ignoring the books and War Servants seems to work out. After killing The Devourer of Secrets, go back to the central chamber where a new raid will start (After the death of the devourer of secrets the invasion of the central chamber continues!).

Answer 2

•  The Scourge of Oblivion

After some time killing the raid, the following messages will appear:

The Masterbook has filled up with the wrath of the library!
The Leader of the invasion finally joins the battle! Beware, the scourge of oblivion enters the fray!

In the central chamber, you will face  The Scourge of Oblivion, which will constantly change between three stages. During the "regular" stage players should deal as much damage do the boss as possible. The two other stages will be accompanied by a broadcast:

• The Scourge of Oblivion activated its reflective shields! The boss will change to a dark red color and reflect all damage it takes, so everyone should stop attacking it immediately.
• The Scourge of Oblivion prepares a devastating attack! The boss will change into light blue color and shoot 4 very strong beams towards all directions (north, south, east and west) that can deal massive damage. During this stage the boss is also invulnerable so attacking it is pointless.

Upon killing it, you will get the  achievement Library Liberator and the following messages will appear at the center of the room:

NOOOO! WE HAVE BEEN FOOLED! THE ATTACK WAS A RUSE!
AMIDST THE DISTRACTION THE KNOWLEDGE ABOUT THE GODBREAKER HAS BEEN STOLEN!
COME TO ME! WE HAVE TO TALK!

• Source: The Secret Library Quest (tibia.fandom.com)
• Also check (Videos): Summer update 2018 Final boss fight (twitch)

Test Server: The Scourge of Oblivion (Final Boss Secret Library quest) (youtube)

Final Boss of Library - The Scourge of Oblivion (youtube)