Dont know how anyone can kill the moon lord solo, let alone in expert mode (yrimir is a god, just saying). I ened up having to cheese the god when i fought him solo.
I actually solo'd him (in normal, anyways) and had a rather easy time compared to most other people. If you have nobody to tank for you, be the tank. I went at him in full offensive Beetle armor (so chainmail), hoverboard, fire gauntlets, obsidian shield, worm scarf (I went out of my way to start an expert world just so I could get this) and the celestial shell (nighttime for werewolf bonus), and just mashed the stuff out of him. Used an Influx Waver (god tier multi hitting beam), Solar Eruption (did most of the melee damage) and vampire knives to heal a little when he doesn't have the debuff on me.
I also had a damn good arena set up, multiple layers of platforms with heart statue spawners all over it. If you can, get a mount with unlimited flight (eg. UFO) and ditch the hoverboard for something more useful. You can still trigger pressure plates wired up to statues while in a mount too. I read somewhere that it's a good idea to set up a solid wall above you to cover under when he does the super-strong laser beam thingy. Also, the sword sharpener and the other buff furniture help quite a bit.
If you are STILL strugling, do the above, but replace melee accesories with ranged ones, get a Phantasm, get some ichor arrows, and promptly hold left click for 10 second while looking at each eye, and enjoy your 4000+ DPS.
EDIT: I forgot: Use all the potions. Endurance, ironskin, regen, hearthreach (if you are using statues), lifeforce, swiftness, wrath/rage, well fed, thorns and flask of ichor + weapon sharpener + ammo box + bewitching table (with the dragon, a summoning potion might be helpful too with it) are what I used.
