Make Attack Rolls
Advantage on saving throws have no impact whatsoever on spells that have attack rolls—or spells that don't require a roll at all (like magic missile).
Scorching ray, fire bolt, shocking grasp, spiritual weapon, and so many more spells don't require a save and are unaffected by any modifier/bonus to a saving throw.
Additionally, weapon attacks work well, too. High Dex magic users can have a short sword or rapier with a decent attack bonus to hit 'em where the magic don't protect. Combine that with the true strike cantrip and you have a pretty good chance of hitting.
Summon Others to Fight For You
There are many spells available that summon creatures, elementals, fey, undead, and more that can fight your enemies for you. Again, no saving throw means no bonus.
Rely on Allies
Having allies who don't use magic also increases your survivability.
Run the Heck Away
While not a way to fight someone, running away from a losing battle means you can be better prepared for the next encounter!
Spells like absorb elements and blade ward can mitigate the damage you take. Expeditious retreat and misty step can get you out of dodge fast. Blur, mirror image, and shield can help with avoiding damage. Blink and invisibility allow you to move without being noticed.