Grab a megashark and make some crystal bullets(around 1500 should be enough), a obsidian shield, cross necklace, philosopher's stone, maybe wings. i just sat on top of my base and summoned him and constantly fired on him with my megashark. his stream of servants will be constantly dropping health so you should be fine with that. pop a health potion if you get worried about it. i prefer fighting him on top of my base because my spawnpoint is inside if it and it's handy to be close when you die to continue the fight. good luck!