If you can't handle her at all the best thing to do is get some antibiotics and crush them up in some wet food. Has she been spayed yet? If you get in touch with the vet who spayed her they'll have a record of her weight . They'll be able to give you the right antibiotics for her.
If she hasn't been spayed yet and is wary of a humane trap you could try a drop trap. Let us know if you need any advice on making or using a one.
If you have a good relationship with the vet, maybe you can get some liquid antibiotic to mix in her food. This is assuming trapping is impossible. If you can't get an antibiotic, there is amoxicillin available from Amazon sold for fish tanks. I have no personal experience in this, having just read about it - perhaps someone with experience treating ferals will chime in.