What to do with a +Def -Res S!Camilla

I'm assuming you mean Summer Camilla and not Spring Camilla?

This is just one of the few/many possible variations that she can be build. This is with the speed boost from Hone Fliers buff. This is probably the most "budget" way to build her, or at least one of them.

Try Raventome and Triangle Adept along with Iote’s Shield and make her a Colorless killer

If it's summer Camilla you mean, the standard builds probably work the same as usual.