He 100% and a 6% driller were the only one standing before I attacked.
I did a little over 3m dmg to him and driller with sylphi osiris jasmine lingling nilya
At the end of the batlle he healed to 100% again... I never seen bosses heal in raids.
Chapter 17 isnt hard but if it takes so long to clear it's better to clear 16 twice in that time... also loot in chapter 16 is better.