2 options
1. Treat it
1.1. Do not put NEGERSENT powder... it has a chemical that when used in over quantities is poisonous to animals.
1.2 Use a spray on like "Topicure" which will stop the flies, maggots and start curing the wound. It costs 550/-
1.3 A feral cat is hard to cure as there's no control over his/her actions. Adopt him/her... take him/her home and treat the wound properly. Put a wound collar so she can't lick the medication off the wound.
1.4. If you are a Buddhist... remember that it say animals suffer for their past sins... adopting, helping and curing a creature like this will have unending good karma on your part. Eitherway... helping someone in need is a quality of humility and humanity. Save the animal.
2. Or.. the other option is find a vet and put her down. As in euthanize cat. Save him/her from the misery.
2.1 You will have to put up with a life of knowing that you could have saved yet you killed an animal though... but that's a different matter alltogether.
conclusion
Adopt.. treat... save...give life... get good karma