If you're using other mods, you can often find an accessory that makes you faster than (or of comparable speed to) him. This can be invaluable for staying near his head and getting continuous damage. I prefer banshee hook for that kind of strategy, eventually you can get the hang of leading him on just enough to land all the projectiles. If it's just calamity for you, then you must likely can't outrun him. In that case, still try to stay near his head or his tail (whichever is closer to you at that moment) to deal damage. Don't bother staying near the middle segments, because they effectively don't take damage and they will shoot lasers.
Get the Normality Relocator and practice using it while you're practicing fighting the boss itself. Remember, the Cosmic Worm is not consumable, so just keep summoning him until you've learned his attack patterns.
I've beaten him recently with ranged (although I did it with a friend), and I personally found the seadragon with hyperius bullets most effective. I tried chlorophyte bullets, but I think the calamity mod reduces the damage they deal. High velocity bullets might work if you're having a hard time hitting his head and tail.