Best bet is to catch the cat in a HaveaHeart trap and let it out in an enclosed space (like a small bathroom) and work with it a few days, so you can then put the cat into a carrier and take it to the vet. if you can't do that, call a feral cat association in your area for help.
You can put the carrier out for a few days with a food bowl inside...get kitty used to associating the enclosure with something pleasant.
With the carriers I use for our girls, I leave them around the place with the doors off and bedding inside so the carrier becomes a safe haven of sorts (even though it is used for transport to the vet).