Im not sure what cast bar youre talking about. Im talking about when he knocks back the tank he is on and is about to one shot him the other tank taunts off. What is happening is kephess is going back to the original tank after the taunt and killing him then going back to the tank that just taunted.
The tanks are being killed by that arcing slash or whatever its called, and i know the taunts are going off because im one of the tanks and its happened to both of us, when i taunt he turns to me then flickers, kills the other tank and comes back to me.
But I havent cleared this place since it was pretty new and it might just be that something has changed or im forgetting something.
after kephess does his knockback, he will start casting an ability. breath of the masters i believe it's called. this causes the tank he casted it on to start dropping purple circles. 4 in story, 5 in hard. if the tank that has this debuff takes any damage, he's dead. KO.
therefore, tank #2 must taunt during this casting bar, so that kephess switches targets immediately after he places breath on tank #1.
and as someone mentioned, tank #1 with the purple circles shouldnt do anything other than place circles, as kephess might do his random attack. and that would result in dead tank #1. from your description of your problem, i think this is the issue. try doing it with tank #1 doing nothing else while he places circles.