Hello Bimdan. New player here.
I clicked on your post thinking I would be able to give some advice but later realized that you were on a higher difficulty setting than I have been able to Victory. After reading through all the comments from yourself, kramaswamy.kr and AzuraWrath I also realized that I would not be able to give advice that detailed.
Originally I wanted to mention that the game allows you to see the Clan Bosses' abilities and Cooldown timers. Reading through those and knowing the order in which the Clan Boss uses the abillites would help plan for the fight in the future.
Currently, I've been facing an easier difficulty level. My Champions have been using abilities that apply shields, heal and remove debuffs. They also buff for ATK and C.DMG to the entire team. Also, HP Burn is doing a lot of damage to the Clan Boss as I believe that debuff deals damage based on HP. Clan Bosses have a lot of HP! As kramaswamy.kr was mentioning. Speed Artifacts help. Faster Champions means more turns but obiviously you could be missing out on other statistic upgrades like, ex: DEF and HP that could give you more turns by keeping your Champions alive longer.
I have similar plans for future difficulties of the same Clan Boss. I forgot his name was Demonlord! Wait a moment, no I didn't. I was just busy thinking in a different area of my brain.