Make a full damage build or damage/dodge build (You get one shot anyway,Dodge item like black belt can give you a small chance to survive the instakill) , get a perma flying item like witch broom for good agility to further dodge the attacks, get a weapon with high damage and large area of attack, memorize her attack patterns and retry at least 20 times. I advise to beat her post Martian Madness/MoonLord. And if you dont care about beating her legit I think the flaregun bug is still working (you basicaly shoot 1000 flares in one spot reaching the limit on projectiles on the screen making empress unable to shoot any projectiles making a fight 100x easier,she can still bump into you tho)