!! CONFIRMATION FROM DEVS !!
(formatting is going to be a bit weird as we try to figure out the buggy solution. Apologies.) I received email confirmation from Dead Mage saying they are looking into the bugged achievement.
Dead Mage and 11-Bit confirmed they replicated the conditions we have found here. They are working on a fix and will update eventually.
POSSIBLE NEW SOLUTION WITH MORE CONFIRMATION
Arkzein in the comments mentions:
John's last family trait (level 20) is auto-regen for the family. I noticed the heal icon above my head symbol every time I started a world. I reset his skill points to turn this off and on the first try afterwards I got the achievement. Worth a try if everyone else is struggling.
According to crazyeyesbtb:
I just unlocked it, after beating it multiple times and not unlocking I found on the reddit that uninstalling and reinstalling fixes it. I did that, ran through 1-1 with Lucy and best boss unlocking achievement instantly
This was further checked by KingOfAttolia:
Seconding what crazyeyes said, I reinstalled it on Xbox and it unlocked first time. [...]
Arkzein gave a similar account when homie says:
Thinking about it I had switched from Xbox to PC when it unlocked for me as well, so not a reinstall but similar. (Though I attempted a couple of runs with no luck on PC before resetting John and Unlocking it). May be another variable, it forces a gamesave sync when you switch between platforms, might reset the flag of whether it's unlocked or not? Frustrating when there's no clear reason or solution. I also avoided even stepping on health pots or picking up any charms / graces etc. just in case. Jut a straight run to the boss.
Definitely try these things! Would love more confirmation on John's regen skill screwing up the achievement. Fuuldrin in the comments mentioned always having damaged healed even though Fuuldrin did not receive any damage. This seems to link up with John's regen ability.
I would also love more confirmation on reinstalling the game or switching between platforms. I completed the 1000G on PC before the update so I cannot speak to switching.
Thanks for everyone's input!
The solution appears to be:
Check if John is level 20. IF John is 20, reset his skill points and do not spend any. Use another character and complete the whole dungeon on NORMAL without taking damage. [Dungeon being defined by a set of floors ending with a boss; so NOT all of world 1 or 2 but rather world 1-1 counts as a dungeon, as would world 1-2 and 1-3 etc.]. If this does not work (and we have confirmation that this may not work), uninstall and reinstall the game then go straight for this achievement.
The reason for doing it on normal is because the game did not launch with hard mode and it was the update that added hard mode that broke this achievement.
What I do not know is if getting hit with Lucy even with shields up voids the achievement since it's technically taking damage, even if it's zero damage. I would like more confirmation in the comments below
According to smmhsmblmjmm in the comments:
In case it's helpful, this unlocked for me the first time I concentrated on not taking any damage in the whole dungeon (used Lucy with the 3 shields in 1-1).
This seems to indicate losing shields as Lucy does NOT void the achievement.
According to Everpeace6255 in the comments:
Smmhsmblmjmm's solution works! complete a dungeon without taking damage.
Further verified by Pandriarch in the comments:
I ran through a whole run with zero damage and also got the achievement
!!Need more verification, keep the feedback coming in the comments below!!
Below in the spoilers is the initial problem:
!! The OLD solution starts in the spoiler below !!