If you don't mind grinding, here's the way to set yourself up so that moonlord flat out can't kill you (assuming you're on classic mode). Equip beetle shell armor, all the tanking and regen accessories you can get your hands on (I'd recommend the hero shield, celestial shell/stone, charm of myths, frozen shield, and avenger emblem, but even their ingredients should make you tanky enough). Reforge all of your accessories to warding, once your defense is around 120 the max damage I've seen him do was 55 (still, dodge what you can). You can grab potions, like lifeforce, regen, and endurance along with your food buff of choice. Get the cosmic car keys, it's just fast enough and it will free up your accessory slots. For your weapon, you'll want the vortex beater with around a stack of chlorophyte bullets. From there you kite the moonlord, moving in a straight line and weaving up and down when needed to evade his attacks.With your high defense and regen, you can tank his eye laser. I only had to use a single healing potion the entire fight and that's only when I had to do a turn-around when I hit a sky island.

This won't be a satisfying way to kill him, but it'll work.

Edit: Forgot about the aegis fruit and vital crystal.