The bosses in 12Sky gold are very hard at the level they are given to you. There are two ways to take these guys down without being killed over and over.

First way is to wait a few levels before killing them. This isn't always the best way as you can fall behind on the quests.

Second way (and the preferred way) is to find someone else who has to kill the same boss as you. When you do GET IN A PARTY with that person and kill the boss together. This way you BOTH complete the quest when the boss is killed. If you do not party and both go to kill it, only ONE will get the quest completed.
Note: The 2nd way does not work with Golden Tower Lifters.

Zombie Soldier Moku – Gungsong Fortress (8460, 7330) lvl 14 quest, located east of town by the large tree
Swordsman Hoyu - Gocheon (3570, 9053) lvl 28 quest, located in center island, southwest corner just in front of the camp
King of Poison Finger - Mujung Palace (3033, 1087) lvl 32 quest, located near the end of the cave, in the last “hall” before the room the Chaser is in
Pirate King - Gocheon (1960 2590) lvl 36 quest, located to the west of the west bridge
Deity General - Tanwak Forest (6134 ,7451), located behind the East Temple near the center of the map, by an old Bhudda statue
Emissary - Gungsong Fortress (7244, 6328) lvl 54 quest, located on the building behind the Blade Trainer
Black Robe Somok #1 - Gungsong Fortress (7012, 8474) lvl 56 quest, located on the south wall in town
Black Robe Somok #2 - Gocheon (9022, 1547) lvl 58 quest, located just south of the portal to Gungsong
Vagabond Jin - Tanwak Forest (3326, 1127) lvl 60 quest, located in the back of the building in the north