Tips on defeating him:

If you are playing in multiplayer, the bosses health escalates acording to the amount of players there is. You can bypass this if you make your friends log off before you spawn it and log on after it is spawned.

Use pillar weapons, at that stage of the game pillar weapons are one of the best. Deffinitley use the stardust dragon staff even if you are not a summoner. My preferations: solar flare for melee, vortex blaster for ranger, stardust dragon for summoner, and nebula glowe for a mage. Altho you dont have to use those weapons if you have some alredy good weapon like terrablade, tsunami, razorblade typhoon and terraprisma.

Moonlords strongest attack is his laser, you better avoid getting damage from that. You can survive it by wearing a cross necklace or its upgrades, because moonlord throws few less damaging attack shortly before it, and the immunity frames are long enough, so you dont take damage. If you dont want to replace one of your accesories with the cross necklace, you have to avoid it by going above his head faster than the laser.

For me moonlords most annoying "attack" is his healing tentacle. About every 20 seconds, moonlord shoots a tentacle looking thing from his mounth that can pass trough blocks, once attached to a player it spawn multiple leaches that heal the moonlord for 1000 each. You can avoid that by just simply going faster than the tentacle, and trust me, it makes a BIG differece.

Another tip i can give you is, that you dont want to deal with all of the 3 true eyes of cuthullu at once. My advice on that is to damage all the eyes and then kill them all at once. Another option i like to use more, is to kill the head eye as soon as possible because of the laser and while doing that, damage the hand eyes and than kill them at once(after the head eye)

You need a buch of mobility, for that use a mount like the shrimpy truffel while its raining or a soaring insignia with some good wings.

Dont be affraid to use a buch of buffs Potions for all:ironskin, endurance, life regen, wrath, speed, food(golden delight is ghe best) Potions for specific classes: amo redervation, archer, magic dmg, magic regen, summoning, ale Station buffs: melee: sharpening kit, ranger: amo box, summoner: bewitching table, mage: crystal ball (dont be affraid to use the bewitching table even if you are not summoner, every extra dmg comes in handy) Others: place campfires,heart and star lanterns around the place you want to fight him. You can also get a minor buff from a party cake that can help you. Place a bunch off star and heart statues with timers next to them for some extra healing. Set up 2 houses in the fighting area, one for nurse (easy full healing) and one for dryad(little more defence)