If you're after completing this achievement ASAP, open the spoiler. If you are after enjoying the story without spoiler, just continue playing as you would and this will eventually unlock.
*** Spoiler - click to reveal ***
You' ll meet up with Jack and the rest of the team. Since the warden's been fired, the whole place is a mess. Grab either the iron enchanted sword or the diamond sword from the armory. After, you'll encounter a lot of enemies to fight. First, a large group of zombies, spiders and creepers. Afterwards a large super ghast with a large number of regular ghasts. Keep slaying the fireballs back at the ghasts to damage/kill them. Eventually, take down the super ghast.You get an option afterwards to help Xara or to let her handle her own stuff. After that's finished, the achievement will pop.