And most mages beat any rager if they're prepped right. Let's go through the classes:

AP: Hardest road to follow, but sleep will get there. Just don't try to fight the ragers straight up unprepped. If you're in a fight, crippling them helps a great amount. Iceball spam plus haste sometimes gets you there if you're full ABS+SS, which is what you should be doing if fighting RBW's. Also, things like being fire for resist and a medium to big weapon helps a ton. After a while, a rager seeing you will run away fast. Oh, don't forget sigil.

Shifter: Next hardest, but completely doable. Admittedly, this is more of a hit and run unless you got a specific build, monkey being the best IMHO. Not happenning probably unless you're air/off or air/util probably, but if you're those, full prep and careful watching from the skies will let you pick them off easy when they stray from the loop. Heck, if you're around with the rep, ragers are not leaving big D cause they can't really do jack against ya.

Necro: We're talking 51, not 47. If you're 47 and trying to fight RBW's, you're doing it wrong. Sleep is your friend, but army helps a lot too. Luring ragers to graveyards also works. There's one in the ruins and someone has a summon spell. Summon behind centurions can get there even at 47.

Invoker: Quicksand kills like 70% of ragers. I don't really care if you have to run off and pop speckled three times, but it's death to most. Outside of that, immolation kills all non-defender ragers and last time I checked, there were like zero defenders. If you're fully protected, and fighting one-on-one, you typically don't care about their injures while you do Rain of stone and other crap in electricity shield. I actually like any competent full invoker's chances in a cage match.

Trannie: You tank mad with a staff, but can't let it out of your hands. They can't hit ya with their DB's while you provide some loving. If the spells aren't working, c word and try another time. You always have the option of duo;follow rager and see if you can catch him with the pants down. I don't think they kill you if you don't let them. Same can't be said for most of them.

Conjurer: The rager killer IMHO. A devil straight up owns ragers, an archon means they can't actually kill you, an angel is extremely nasty to evil ones. Even a good demon will own up a typical rager. Strategic familiar and clair use and making sure you don't fight unprotected goes a very long way.

In most cases, having ABS+SS plus cast word means they can't kill you. If things go south, you just get out. Thing is, if things go south for them, there's no getting out. Quicksand + vortex on Eastern? Dead rager. Flyto/monkey smash a bit away from village? Dead rager. Lucky neuro + disrupt spam with strategic slow? Dead rager. Sleep? Dead rager. Not like he can teleport away. Devil supps? Dead rager. 2K hp in ABS plus bash? Dead rager. Honestly, I don't see a mage losing if he cares not to.