It seems scary tough, but with post-moon lord gear, it can be super easy. buffs and stuff can help a lot too, of course, - perticularly lifeforce potion! - and, of course, things like stardust dragon can help too. I used full nebulla armor with nebulla arcanum, magic power, mana regen, ironskin, lifeforce, regen potions, and a food that gives lvl 3 well fed such as seafood dinner.
heart lantern, star in a bottle, campfire/fireplace, buff stations can help too.
The battle was over suprisingly quick XD
Well, it was on normal mode though. It would be surely tougher on other modes, but, since all worths the exact same achievement... might be a good idea to try it on normal mode.