End boss of Borderlands 2 with a Legendary Conference Call shotgun and The Bee shield.  It's just utterly ridiculous.  (not my video but it's essentially the same)

World of Warcraft.  I remember playing the AV battleground and running into a world famous Warrior (seriously, his videos were mega-popular).  I had decent gear but the people at the top had crazy good stuff, like far beyond what a normal person could get.  Besides that, I was a Rogue and Warriors ate most Rogues.

So, he's sitting down and healing at the back of the battle, right?  I'm nervous but, seriously, this is my only chance, so I go for it.  Backstab!  I start doing my routine on him and ... it's like attacking a moutain.  Just, seriously, it's immediately obvious that I'm doomed.  But I've got crippling poison on, so he can't run at me, but then, if I get too far way, he can use a skill that charges right at me from a distance.  So, I start walking backwards, slowly, while he's slowly walking toward me.  I'm just delaying my death, though, and we both know it.

But then, at it turns out, I drew him far enough that some high level NPCs got aggro on him and came charging over.  They're WAILING on him but he's ignoring them in his effort to get revenge on me.  I won't let him get close enough, I just continue to stay out of reach, and they eventually kill him.  It was pure luck, sure, but it was also hilarious.