Two questions:
Are NPCs eligible for godhood?
For challenges it’s possible to get an NPC to do the J-9, kill a chicken walker, clear 5 levels of a slime pit, kill a thriller with a thriller novel, kill the hulks from the thriller killed with a thriller novel, kill a shocker brute unarmed, and/or kill a fungal structure. If NPC needs are enabled they could probably drink a barrel of whiskey too.
For milestones you could give an NPC a full set of fancy clothing, and give them an artifact.
And, is there something I’m missing with the slime pit challenge? I tried using the debug menu to explore several map tiles in a test world; none of the dozen or so slime pits I found went down to 5 levels, and each level on the overmap did not look like the slime pit from Nijuni’s screenshots.