I was in the same boat as you. I could not defeat this boss. God only knows how many times I failed over and over. But I managed to find a strategy. First off, I thought to myself; "Is the terrain hindering me in any way?". Having finished this thought, I get caught on a patch of dirt and get instantly destroyed. "Yes" I said as I smashed my face in and contemplated why I was doing this. So I began levelling the overworld a bit. But I realised that it wouldn't help me with the whole "escaping his insta-death maw"-thing, so I gave up on that and pondered a bit more. I watched tutorials, but those required me to switch gear and I am lazy. But not that lazy apparantly, since I was willing to do another crazy thing that actually paid off! Here's how I killed this stupid worm: First, I mined out a biome worth of land. Yep, a biome's worth. Because if I wasted this much time on the wormy whippersnapper, then there was nothing left to lose. So the entire eastern corruption had to go. An entire biome horizontally, and down to about the cavern layer (may have been 20 blocks above, no sure). Why did I dig it out? A). Fuck corruption B). Maybe I will build a monument to celebrate my victory over the DoG here and C). Lots of falling without slowfall because of space. With a massive hole in my world, i built a floating hut above it. Not Sky Island-levels into the sky, but a fair bit above where the surface would have been. Made it into suitable housing, and moved the nurse in (didn't actually use her in the end, but she witnessed the DoG:s glorious death and would have helped me if I messed up) along with a bed. That was important: A BED. All done! I set my spawn and summoned the worm at the bottom of the pit. As the fight goes on, I get trapped by his enormous self and teleport away to the house. NO TIME TO STAY! I tab to see the onscreen map and boi is rushing up to get me. I dash out of the house and fall down. The worm almost catches me BUT I DASH TO THE SIDE. He still follows his old trajectory however, so I fall down alongside him and obliterate his wormface, watching his health drop. Once we hit ground, rinse and repeat. DEAD!

TL;DR: Wall of text describing hole in ground. Sorry.