Hardest video game boss you ever faced ?

[XVIII]

Mines has to either be Shao Kahn from MK because he don't get stunned and can kill you quick or Mike Tyson from Mike Tyson punch out which basically explains itself.

 

[fiend]

Ornstein and Smough in Dark souls.

Especially after killing Smough first, the powered up Ornstein in NG+ took me about 16-20 hours to beat.

 

[BadBoy M]

Mecha Tank from Legend of korra
i died like 20 times to beat it