SRD said:SPELL FOCUS [GENERAL]
Choose a school of magic.
Benefit: Add +1 to the Difficulty Class for all saving throws against spells from the school of magic you select.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new school of magic.
SRD said:ABILITY FOCUS [GENERAL]
Choose one of the creature’s special attacks.
Prerequisite: Special attack.
Benefit: Add +2 to the DC for all saving throws against the special attack on which the creature focuses.
Special: A creature can gain this feat multiple times. Its effects do not stack. Each time the creature takes the feat it applies to a different special attack.
Henrix said:Augment Summoning has an effect very similar to Spell Focus, it augments creatures conjured with a summon spell.
I'd say go ahead and invent a similar feat for your demons, but which works with spell-like abilities.