Arenanet developer Crystal Reid has tweeted out in response to some questions revealing that Ascended gear will be required for some raid bosses
The tweet states that early wing bosses can be killed with a mix of Ascended/Exotic but last boss will require full ascended. There is no agony mechanic like in fractals.
@blacklionchest Earlier wing bosses can prob be killed by top tier players in mix of asc/ex. Last boss should be full asc.
— Crystal Reid (@All_Caulle) September 30, 2015