PLEASE fix that Naksun nonsense

[Ratella]

Mushin tower : floor 15

Server: Greenhollow

It happened to me on several occasions already. I get Naksun to 1hp with 10 seconds left on the timer. i continue hitting him, over that 1 hp, but Naksun continues on fighting, doesn't die, gets enraged at the end of the timer and kills me. Its already hard enough to kill him with 536 AP and survive the third phase. But this invulnerability is just plain nonsense and a sick joke. How am i supposed to kill him? Am i missing something here? 

Please Fix it because i cannot stress enough how frustrating it is.