To get this achievement beat Kallamar like normal. Don't worry about not getting hit the first time you fight them.
When you're able to run through Anchordeep again and you have the option to continue crusading on the same run after you've beaten whichever mini boss, this is when you will start setting up for this achievement.
Go through the crusade run until you feel like you have a lot of good tarot cards, a high damage weapon preferably with poison damage, and a preferably ranged curse. I also recommend having fully upgraded the Temple devotion reward tree. As this will help you vary your curses and increase your fighting stats. A good indicator would be being able to quickly and easily beat the mini bosses.
I focused on getting tarot cards that would increase my curse damage and reducing the amount of fervor cost. That way, when I entered the boss fight again, I could spam the ranged curse. I typically waited until i got the curse that seeks out enemies when used
(Side note: attempt this method when you aren't too concerned with your followers' faith or food levels as you might be gone for a while with this method)
Once you feel confident in all your tarot cards and fighting stats are good, select a crusade run branch and the first room you enter should have a statue of Kallamar that you can steal devotion from. Break that statue and enter the red portal. This will start the fight with Kallamar again, and if high enough level, with a few hits, curses and dodging you should be able to get this achievement.
Also I used this method on easy difficulty with the Fleece of the Glass Cannon.