How to take a -1 penalty to CL for a single spell?

[Nettlekid]

Just a quick question this time. A character of mine (actually a cohort) is a slightly watered down Cheater of Mystra, and I can't think of a good way to make it so that when she casts Antimagic Field, it's one level lower than her actual caster level. If it's one level lower, I can use Arcane Mastery to take 10 on the caster level check to cast in the AMF, so that if the AMF's DC is 11+CL-1, then 10+CL will make it all the time. But I can't think of a good way to do that. I'd prefer if it could be done with a feat or with a spell, not items, please.

[Emperor Tippy]

A caster can always cast a spell at a lower CL (minimum CL is the minimum required to cast the spell), you just have to do so. Nothing at all special need be done.

http://www.d20srd.org/srd/magicOverview/castingSpells.htm#casterLevel

Second line down.

"You can cast a spell at a lower caster level than normal, but the caster level you choose must be high enough for you to cast the spell in question, and all level-dependent features must be based on the same caster level. "

[Nettlekid]

Oh, really? That's good to know, for all sorts of reasons.