Theres only 3 ways to be a mage killer:
1) Grapple them. This is where monks come in.
Currently my Druid/Barbarian outshines the Monk by a long shot when it comes to grappling. Still low level of course, but even higher level, I think I'll give her a run for her money. Raging, Improved Grapple plus Bull's Strength gives a really solid boost. The party mage has also jumped on the bandwagon with Enlarge Person and Ray of Enfeeblement on enemy grapplers. The DM hates us at the moment.
I also started adding poison and contagion into the opening touch attacks just for fun. Throw in the later ability to shape shift into a Large bear at some point, it gets even sillier. Earth elementals after that

Not to mention the spell casting flexibility you gain over a monk.
So far, it's been a good mage killing group as well. We have the Bard with silence, glitterdust and other spells, and the Rogue sneak attacking into the grapples - we have come out on top in two encounters with much higher level NPC mages.)
Aside from that I'd recommend cleric / bard, specifically to re-re-iterate the Silence approach.
As NPCs in the past I have used counterspelling sorcerers, but they may need support to finish the job if their focus is countering. Depends on how you build them.