What ways are there to make the DC harder for an enemy caster? I know of disruption for fighters and the arcane bloodrager gives them a +2 to the DC. Are there other ways to make it harder for a caster to cast?
Anything which penalises an ability check - conditions like shaken etc.
You can add more concentration checks with some battlefield control spells, ongoing damage or readied actions.
The Spellbreaker archetype for the Inquisitor has a lot of toys for this, and the Spellkiller inquisition grants them Disruptive without fighter levels.
Between Spellbreaker's Foil Casting, the DC increase for Disruptive, and the increase when you hit them, you could get a +6 increase to their DCs.
The tricky part is figuring out how to threaten them even if they five foot step away. Step Up is your friend for this.
You can also ready your action and strike while the enemy is casting. This is one of the premier uses for Vital Strike.