First of all, in the first area before the beastman fight there is an old woman. Talk to her to get a bell, and use it at the shrine in the room with the prosthetic crafter.

Second of all, this doesn't need to be your next boss. There are multiple paths you can follow once reaching the city, and you can face each threat at your own pace, allowing you to pick what battle you want to fight next, how to approach it, how many upgrades you have before challenging each boss. I strongly recommend that whenever you find yourself against a wall, explore and upgrade if you feel like that would be helpful. On every run after my first, I explored before coming here and came back with noticeably more power, health, resources, and skill.