In Dark Age of Camelot there are three realms fighting each other. Each of these realms had a dragon to fight against in the far ends of their homelands.
I was playing Albion that time when these dragons were not even beaten on every realm, not on mine. It took us hours, hundreds and hundreds of players alone to reach this open world boss and to give him a proper fight. When we won the loot distribution of the (realm-)raidlead took more than half an hour!
It was an insanely good community back then 
The longest boss fight on my own was probably in TERA where I had to fight a super - BAM (the game names these types of npc “bad ass monster”). Some sort of BOSS BAM. Me, slightly undergeared and two or three levels below him. He could oneshot me with his “charge” and other skills I was not allowed to die at all. 1hour and 31 minutes i wont forget :rolleyes: