ah! i missed the 1/2 HD for the SLA DC, good catch.
i was estimating the CR should be around 10-14, but then the problem comes in that it casts spells as a 20th level caster. it's got some nice ability scores, SR, and a built-in magic weapon.
the CR could be 17, or even higher, if you are thinking spellcasting level = CR +1. but i don't quite think it rates an epic or near-epic challenge.