They're squishy casters by and large, and unfortunately Tethys gets punished for having poor health in almost all of her builds, so Sweep punishes her incredibly hard too.
She likes anything that can give her armor, and frankly, sometimes it's better to put her on the second floor rather than the first, so you have time to get rid of Sweepers
Don't underestimate their Lodestone monsters either. They sit in the back and usually don't do anything but give you card acceleration or other effects on Incants. Those can get very powerful, and if you upgrade them with extra health they can be walls for Tethys in a pinch.
stacks on stacks on stacks on stacks