took 5 tries but i finally solo'd the moon lord in expert mode. had a minecart track spanning the length of my world and chlorophyte bullets and stufftons of buff potions. it took a while and i got to 26 hp but i beat him and got the S.D.M.G.a allowing me to beat him easier next time ;)
What i've noticed, and you all should have too
Every single boss becomes dough after you beat them once
They drop that one weapon that just breaks them to pieces (if you have skill of course)
It's always hard the first time (Unless its a boss before finals with maxed out characters)