I know that in a normal world I can spawn blue slimes on top of me, so I take 1 damage to render me invulnerable to any other attack. With a campfire and a heart lantern nearby and standing in honey it's possible to tank the moon lord in a normal world for an infinite amount of time.
Does this also work in expert mode? Seems one hell of an easy way to beat any boss. The only problem would be killing Skeletron as you cannot get the mechanic before entering the dungeon.