All the SLAs are DC based - evocation DC and anything related to eldritch blast (aura, bursts, slas, etc.) is treated as a level 9 spell for DC purposes based on a prior message from Varg.
I currently only use tainted scholar for mainly the crit damage boost and pact damage. Basically you can get your DC high by maxing charisma. At epic levels you have some other options for buffing DC or debuffing enemy saves.