You must have the Dark Lance, unless you don't have it.
It's a good tactic for first fights, like me (but i played on my bro's phone).
It was my first boss, i just defeated it in minutes tonight.
I loved fighting it, so maybe i could defeat Skeletron (my bro defeated it).
I did this with no buffs, potions or self healing (i've seen someone do this with the Pumpkin Moon).