The maze was built starting with the mother skeleton replenishing its magic.

On the 36th day since Lance was reincarnated as a skeleton, a talking female skeleton spawned in the dungeon. "Watch out, newbie. I'll only teach you how to fight once." "Yes, senior!" "When facing an enemy, first raise your sword with both hands." "Like this?" "That's right. Get into position and observe their movements, then... charge!" "Eh!" "That way you'll die with some dignity, at least you fought back." You've learned how to fight a skeleton. Now, go and take down the red dragon in the opposite dungeon. PS: Not a mass-production strategy, only creating weak monsters. As for why, ask Miyazaki Hidetaka. Bring my razor blade elements: [Non-human protagonist] + [Dungeon farming] + [Monster daily life]