User blog comment:Dewven/Hard Hardmode/@comment-216.73.72.107-20130606173404/@comment-10343067-20130606193247

Which means equip cross necklace, get +=40 armor, and hook yourself to piece of hellstone or meteorite, or use cobalt (not obsidian) shield to stand on it. You'll get 1 dmg every second but you are next to invincible to the boss (some hits may get in but its rare). Now spam with you most powerful weapon (Glaive, Magical harp or Dao of Pow should do the trick most succesfully).

I haven't myself tried that trick but those are the basics for it.