I want to increase the DC of the hexes my witch uses. Mainly this would be evil eye and deep slumber. As far as I understand spell focus didn't work because it states that only spells could be enhanced. Are there other possibilities?
I just finished playing a witch in Carrion Crown, and here's how I augmented her hexes (full character sheet). I got her Misfortune hex up to DC 27 at level 14.
The only Feat that I´m aware of is the Ability Focus Feat. But since its a Monster Feat it might be considered illegal for your character.
The bard cantrip lullaby boosts sleep DC's by 2 if they fail that save (and also a -5 perception), and you can take the cantrip with Two World Magic trait.
Slumber Hex DC is also affected by the Elven alternate racial trait Dreamspeaker.
Another character creation option to maximize hex DCs is to create a scarred witch Doctor. This class uses Constitution in place of int. As patron you choose water witch patron. This allows you to use elemental body for a +8 bonus to constitution. This will add 4 to your DCs.