Queen Machine

This one wants Mechdusa - the fused form of the three mechanical bosses, exclusive to a world generated with the secret getfixedboi seed. There, craft Ocram's Razor by combining a Mechanical Worm, Mechanical Eye and Mechanical Skull at a mythril/orichalcum anvil, use it at night, and fight all three mechs welded into one hovering horror. Kill the whole assembly (Skeletron Prime's head is the last segment) and it pops. Fighting the three separately-summoned mechs at once is the different achievement, Mecha Mayhem.