#12? Maybe? Sometimes, a simple pincer attack can make all the difference. Even kobolds can threaten a caster given the right opportunity to get 5 or so on 'im.
Throw an enslaved troll in a pit trap with a trap-release muzzle on him, and all kinds of low-level monsters are threatening again. Especially if the troll has a couple of key common "Spell Immunities" on him. Throwing magic-item equipped mooks at your players is good in the short term, but that junk can add up fast, and your PCs will be throwing that back in your face before long.
#14 Enemy rogues and rangers leading ambushes. Have a unit of baddies hidden by an enemy rogue, with orders to keep quiet until the signal is given. The spot/listen checks to find them will be harder, and PCs will have baddies from multiple directions coming at them. Add liberal fire-support for a real challenge. As an anti-caster bonus, have a couple of thunderstones or rocks with Silence cast upon them distributed among your ambush party, and have the back rank hurl them around the party. That should help. Just make sure the ambushers know that the signal will be visual, since they won't be able to hear anything.