if she is so timid do you think it wise to let her go outside?
I have 4 cats one of whom is very timid, occasionally she'll put a paw out of the kitchen door but no farther than that. You could try one of those Feliway plug in devices that are meant to help keep the cat calm, though I've tried them and found they had no effect at all on my 4. I also never use frontline on any of them either as it seems to irritate their skin and a couple of them have been so bad that they have scratched the skin red raw, pulling out all the fur on that spot, so I don't put anything on the cats at all.
If you do manage to catch her, try and wrap her snugly in a towel to put the stuff on her. Otherwise, make sure you wear long sleeves.