SINNER: Sacrifice for Redemption

Following conditions are considered exploiting boss weakness:
(1) Attack him while his stomach is open. Example: after it spits three purples stuff at you or when he rolls into ice block and stunned for 4 seconds.
(2) Attack him while he is healing (He opens his belly and you see something green swirling inside)
(3) Attack him while he is dazzling. You can check the video at 0:18 and 2:49 to see the dazzling animation.

- After he spit the purple stuff at you, he become exhausted and his stomach opens for around two seconds. You can stand near him and pay close attention. Immediately when he closes his stomach, you can score a hit or two. Same thing to do when he is stunned after rolling into ice block.
- After losing around 50% of health, he can heal himself. Don’t attack while his stomach still opens.
- Avoid attacking him when he is dazzling is the most difficult thing to do. You know he is in a daze when you see black blood splashes above his head. He will be stunned for a few seconds. It can happen when he receives consecutive hits from your sword or when he takes two or three hits from your spear throws. That means repeatedly mashing the attack button may sometimes ruin the opportunity to get this achievement. Example: He may be stunned and in a daze after getting hits, but because you cannot cancel your attack, you will continue your combo, thus voiding this achievement. In this regard, using long sword is easier to manage than short sword.

Using spears is ok as long as you don’t hit him when his mouth opens or when he is dazzling. But don’t follow up with any attack if he became dazzling. Anyway, I don't suggest using it.

If you find it difficult to navigate and die a lot from falling, consider changing the camera setting to Fixed camera. The reason is the Free camera will zoom in too close to you, which make it difficult to see what is behind you. The fixed camera seems to provide better overhead view and may be useful for this boss fight.

Beating this boss without these conditions is already a challenge. And with these conditions added, it becomes very hard especially when the cheap boss keeps healing himself over and over again (you are not allow to interrupt him). So I recommend doing this boss while you still have all your perks, it is worth it to retrieve back your 'sacrifices' before attempting this achievement. Prepare to be frustrated. Good luck.

Credit: The video is not mine and credit goes to the creator - Pascal Desrochers.

Basically the tactic is not to attack when Camber Luce opens the belly, under no circumstances should attack at this time.
I hope it helped you.

Translate for portuguese:
Basicamente a tática é não atacar quando Camber Luce abrir a barriga, em hipótese alguma deve atacar neste momento.
Espero que tenha ajudado

Dont use SPEAR for this achievements !
And of course don't hit the boss when his belly is opened.
Try to stay as close as possible and dodge a lot, specially at the end of the fight : that way the boss wont regenerate.
Hope it helps