Make sure to stay under him and not too far away from him, otherwise you will have to resummon him. Just keep shooting/attacking him as you drop and rise over and over again. Ooooooor the most easiest way is to invite me and i'll dessimate (destroy) him in less than a minute! I can fight all the hard mode bosses at the same time! It's fun! Don't worry, it won't be long till his trophy plaque is hangin on your wall!
He is SUPER EASY!!! But at first he wasn't. Here's a little hint!! Build a Hellivator and before you summon him make sure to have LOTS of health potions, iron skin potions, featherfall potions, gravitation potion & regeneration potion. also use the crystal ball right before summoning him. Just keep shooting/attacking him as you either are dropping or rising.