But it is that easy. It's been made clear on several occasions that the Forsaken are at maximum readiness while they're around each other. Even very powerful Forsaken like Sammael, Lanfear and Rahvin fill themselves to the brim with the One Power when they're around each other. They prepare counter weaves and hold them in place and watch each other intently. It only takes one slip, and I do believe Aran'gar may be the first true example of this that we've seen in the series. She did the worst thing possible - she allowed herself to be distracted by Rand, turned away and allowed Graendal to be placed behind her. Such a small thing is all that was needed for Graendal to have the upper hand, and ultimately led to Aran'gar's downfall. I do think that many Forsaken fell in such a casual manner, which is why channelers like the Forsaken survived. They never ever let their guard down. It must be very stressful.

If they're ready around each other, then none of these Forsaken can take each other on, but even Rahvin alluded to the danger of having Lanfear behind him. He could very easily have died in that scene had Lanfear not wished to speak with him. He didn't even know she was there until she spoke. Aran'gar is the end-point of what I've always suspected would happen to someone like that if they became too comfortable around Graendal.