There's a couple of other problems with some of the suggestions - the Image spells (as Figments) can't make something seem to be something else, or to not be (that's the domain of Glammers) - so you can't Image the BBEG into looking like you, make a pit that's not real, and so on with the standard image spells.
That said...
Messing with the heads of people with Spellcraft:
Quickened Image of the spell you're wanting to cast, Silent Stilled spell you want.
You can't identify a Silent, Stilled spell unless you see the results. A Quickened spell takes no real time.
For example, you put up an Illusory Wall of Stone that is easily identified by a DC 16 Spellcraft Check (silent Image) on the part of the opposing Wizard. At the same time (by way of a Quickened spell), you put up a REAL Wall of Stone (which can't be identified except through the effect). Wizard correctly identifies the illusion, and shouts to his friends... who trustingly run headlong into it.