The timing sequence for Damage Reflect for Empress Cygnus.

That's because it is.

When you attack her solo, she casts reflect as defined in the game files, roughly 80 seconds from the last cast, but when attacking as a group, she can pretty much reflect whenever the hell she feels like, sometimes spamming in rapid succession, with no rhyme or reason, making it impossible to time. We've chalked this up to just her being glitched.

then how do u all kill her in a squad setting?