Sir Black Tea
Dell Conagher
Those boss battles where you're not sure if you're doing enough damage, or any damage what so ever.
Or when they're overpowered and can heal themselves. emotion_donotwant
Or when you JUST got the hang of their attacks and think "yesss, I can make it!" and then after losing 90% of their health, they completely change their fighting style and kill you at once. emotion_awesome