Iron Crypticle

This is quite easy to do on any New Game boss once you've gotten some experience with each boss and learned their attack patterns. The most important thing here is making sure you don't have a weapon pickup active when you actually enter the boss room. If you have any left, expend it before going through the door. Then, it's just a matter of whittling down the boss's health with your default weapon. On Easy, this won't be bad at all, but it might even be easier to do later in the run when you have more stat upgrades. For example, I did this on the fourth floor boss, the Hexapod, with no issues whatsoever because I had enough damage and fire rate upgrades that I did plenty of damage.

After experience beating a run, this achievement will be no problem to mop up if you don't do it naturally. Below is a video of me beating Boulderface without using a weapon pickup (and without taking damage).

https://www.xbox.com/play/media/6BTHKG29LH