I fought plenty of A/B/S mages as Danntruso including a mummy, a few conjurers, and plenty of shapeshifters. It really isn't as difficult to bring them down as you may think. It takes the right build though. The normal giant/flurry spec isn't the right build to bring down an A/B/S mage. You need to be something that can lag them, and also drop their stats to the point where they are taking nothing but hits.
I even fought some tough invokers as Danntruso, and while I never killed them, I could at least hold my own. I think it just boils down to game experience. The more you play an A/B/S mage, the more you will recognize the weaknesses therein. For an invoker, I can tell you that if you lead the invoker into the right situation, they are far weaker then you may expect. For a conjurer, your best bet is to kill their servants, or at least disable them (blind the archon). For a shapeshifter you're gonna want to boneshatter/artery/entwine/etc. Necromancer you're gonna want to drive/pull them away from their army, then do whatever you can to keep them in place.
It doesn't always go as planned, but from my experience with Onya, I can tell you that there are ways to beat an A/B/S mage even as a non-rager. It is dependent on the right build and right know-how. Having said all that, yes it is difficult to do, but far from impossible.
I'm not trying to start an argument, but I reject the notion that A/B/S is an I win button. Much more goes into having a strong character then zap self, zap self, zap self, win.