Orstein and Smough from Dark Souls... I spent a whole day of tries just to get to beat one of them... and then I realized you have to fight a more difficult version of one of the two xD. At that moment I simply let myself die and started over the next day. Even tough that part was certainly maddening I actually enjoyed the challenge of the fight and after I finished the boss fight I felt an incredible joy
"I've Underestimated the Horse Power from Mario Kart 8, I'll Never Doubt the WiiU's Engine Again"