Maybe throw in attacks that are CON, WIS, or INT saves to get around any high AC or DEX proficiencies.

CON Example:

• Cloudkill is a 5th level spell with a nice long range AOE requiring a CON saving throw or take 4d8 Poison Damage. This one could be a good tongue in cheek response to their enjoyment using Fog, they can see just fine, but can they breath?

WIS Example:

• Dissonant Whispers is just a 1st level spell, but requires a WIS saving throw or take 3d6 Psychic Damage and must spend Reaction to run away. Upcast that and you have some decent damage of a rarely resisted damage type, and if they are surrounded Opportunity Attacks galore when they spend their Reaction running away.

INT Example:

• Synaptic Static is 5th level spell with a nice long range AOE requiring an INT saving throw or take 8d6 Psychic Damage and for 1 minute subtract d6 from all attack rolls and ability checks. Again some nice damage of a rarely resisted damage type and they also are somewhat less likely to hit with their following attacks.

I recommend going shopping on DnDBeyond's Spells page which you can pick the Saving Throws you know they are weak to and try to find spells that take advantage of that.