Moonlord would be no prob with Dash there, just give her a good melee or homing weapon and she can move so fast she could avoid his attacks easily. Of course that is going strictly by game mechanics, I imagine a fight with him in a fic or something would be a lot more difficult/interesting.
Coming soon.
Edited
I see
Terraria’s Moon Lord is kinda-sorta implied to be Cthulhu.