Boned

Talk to the Old Man at the dungeon entrance at night and choose Curse to summon Skeletron. Take the hands out first to open up the head, then circle-strafe it - it only hurts while spinning, and a long horizontal arena at the dungeon mouth helps enormously. Win and the dungeon opens; lose and he'll happily re-summon tomorrow night. Do NOT stroll into the dungeon early unless you enjoy Dungeon Guardian physics lessons.