Boss system:
 The first boss will spawn in CF and after it dies another boss will be spawned in a Shinewood.
After the Shinewood boss dies, in CF, a boss will appear and the sequence goes like this,
CF > Shinewood > CF > Flames of Hell > CF > Shinewood > CF > Bloodline of Life > CF > Shinewood > CF > Blade Storm Mountain,
Total of 12 bosses will spawn in a cycle, 6 of them in Chaotic Frontier maps, 3 in Shinewood and 3 in Heaven's Hell. 

Forest of Scream - Lapan [E] Level 40 Boss drops:

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Land of Ice -  Frost Golem Boss [E] Level 47 Boss drops:

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Dungeon 1F - Kalidan [E] Level 55 Boss drops:

 

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Forest of Pain - Gespen [E] Level 68 Boss drops:

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Tower of Despair - Murad [E] Level 70 Boss drops:

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Shinewood - Devil [E] lvl 99 Boss drops:

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Flames of Hell - Axeman [E] lvl 120 Boss drops:

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Bloodline of Life - Warrior [E] lvl 120 Boss drops:

 

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Blade Storm Mountain - Wizard [E] lvl 120 Boss drops: