These bosses can be summoned by obtaining summon scroll pieces by completing guild boss subjugation quests.

Head to the Guild Missions tab of the Guild window, and search for those missions categorized as [Subjugation]. Among the various rewards you are given for completing them, you will acquire summon scroll pieces for the different guild bosses.

As you and your guild mates obtain the required summon scroll pieces, the Guild Boss Subjugation tab will update, showing you how many pieces remain to be able to summon the boss.

After collecting the required amount of summon scroll pieces, your guild can summon the corresponding boss. Once you select "Find Location" and go to the location where the boss can be summoned, you can select the difficulty.

There is also a limited number of guild bosses that can be summoned per server at a time.
If too many guild bosses have already been summoned on your current server, a system message will tell you that too many have already been summoned. In this case, you can either wait a while, or you can move to another server and summon a guild boss there.

After you have successfully summoned a guild boss, it will be reset on Monday at 00:00 UTC, so do not forget to challenge them with your guild on the weekends!

Boss summon scroll pieces do not reset from week to week. Likewise, available guild boss summons do not reset if the guild does not summon them.

Once you have summoned a boss for the week, you won't be able to obtain new summon scroll pieces of that boss until the next weekly reset.

Contrary to other guild bosses, once summoned, Khan will disappear 30 minutes after being summoned, regardless of battle progression.

◈ Guild's Expedition Region

Unique to Khan as well is the Guild's Expedition Region instance, which lets guilds battle Khan without being interrupted by pirates and enemy guilds.
Once you have all the summon scroll pieces to summon Khan, the guild master or the guild's advisors can move to the island of Oquilla's Eye where Khan dwells and open the Guild Boss UI to enter and create the Guild's Expedition Region instance.

Remember that you cannot be on a Node, Conquest, or Arsha server when you create and access the instance, and that you cannot access it if you have forced PvP active, if you've recently hit or defeated another Adventurer with forced PvP active, if you're in a duel or mounted, if your character is not idle, or if you're not in a "normal" zone such as the Elvia Realm.

The expedition region will remain open for 1 hour.
Guild members can enter the instance when they approach the area in Oquilla's Eye. However, keep in mind that only allied guild members and war heroes can enter.

Once the expedition region instance expires, all characters within will be moved outside the instance.

◈ Strategy

To defeat Khan, you must have the [Guild] Monsterbane Cannon Assembly Kit and [Guild] Monsterbane Cannonball items. This equipment can be obtained from a Guild Manager NPC by exchanging Old Moon combat/life seals, or purchased with guild funds at the guild shop.
Each seal can be exchanged for one assembly kit or 10 cannonballs.

Note that the cannon assembly kit will be destroyed 30 minutes after being installed. When the assembly kit is destroyed, all the cannonballs inside will be deleted.

The [Guild] Monsterbane Cannon Assembly Kit item is different from other types of cannons. You can use stacks to use rapid fire or overcharge cannon skills. You can get these stacks by firing the cannon.
On one hand, the charge time and cooldown are longer than with a normal cannon, but on the other, it has higher damage. More stacks result in higher damage output.
You can enter rapid fire mode when you have stacks built up. Rapid fire gives you faster reload speed and fire time. Using normal fire during rapid fire mode will give you stacks.

When Khan <Eye of the Great Ocean> is summoned, beware! You will receive damage when you step into the eye area the moment Khan appears.

Khan's lightning attacks will preferentially target the Adventurer who did the most damage to Khan.

Khan will summon black crystal seaweeds during the encounter. If you don't kill them, the battle can turn against you, as these will continue to summon protties until they are destroyed.

Destroying the black crystal seaweeds will also destroy the monsters summoned from it. You can acquire black crystal cannonballs, which are more powerful than monsterbane cannonballs, by destroying the black crystal seaweeds.
Using the black crystal cannonball will make the battle a lot easier.

A caution message will be displayed when Khan prepares a strong attack. Use the surrounding landscape features to protect yourself from it!
Luckily, there is no penalty for dying from Khan's attacks, and you will be able to revive in Oquilla's Eye via the "Resurrect at the closest Safe Zone" option if you've discovered the topography of Oquilla's Eye.
Remember this, as the "Resurrect at the closest Town" option will revive you in the nearest town.

Khan will prepare to heal itself once it loses a certain amount of HP.
To stop its healing, you can either destroy the summoned black crystal seaweed or hit it with enough overcharged fire shots.

After successfully defeating Khan, you can swim out to the center of Oquilla's Eye to receive your loot.
You will be able to acquire items to upgrade your ships and valuable treasures of the Great Ocean.
In particular, you can receive Khan's Concentrated Magic, which can be used to craft any of the three types of Khan's Heart, one of the most powerful alchemy stones available!

Entering the designated area in the Stonetail Wasteland allows you to summon the guild boss, Mogulis.
To summon Mogulis, you must complete specific guild quests and collect 4 Guild Boss Summon Scroll Pieces.

Ferrid is summoned at the Omar Lava Cave, in central Mediah. Its summon boss scroll pieces can be obtained by completing boss subjugation requests from the territory of Mediah.

Its attack patterns include an area stun, ranged knockdowns, and the ability to summon lava monsters.
 
To defeat Ferrid, it is recommended to attack its back, due to Ferrid being able to stun you if you attack it from the front.
Beware of the lava monsters summoned after Ferrid punches the ground, as they may overwhelm inexperienced guild members the first time they face this enemy. Due to this, make sure to destroy the lava swamps that summon them as quickly as possible.

Upon defeating Ferrid, you may obtain the item Energy of All Creations, which is employed to craft  Breath of All Creations, a useful buff for Node/Conquest Wars.

The Giant Mudster is summoned at Glish Ruins, in southern Serendia. Its summon boss scroll pieces can be obtained by completing boss subjugation requests from the territory of Serendia that do not require defeating monsters in the same region.

The Giant Mudster is slow, but its attacks are powerful and affect a wide area, slowing the movement and attack speed of all Adventurers in range. Be careful to not be hit too much!

If you see it scattering mud around, it's a signal that it's summoning smaller mudsters. These monsters will explode after some time, so it's advised to kill them quickly or avoid the area they're in.

One of the most powerful attacks the Giant Mudster can do is when it compresses itself and thorns rise from the ground, dealing great damage to all those hit by them.

Upon defeating the Giant Mudster, like with Ferrid, you can obtain Energy of All Creations.

* The content of the game guide may differ from the actual game content, depending on the update and content changes.