I'm probably the worst one to give you advice, but since I am starting tonight (
) I can tell you what my mentor told me. I don't know if it applies in your situation, but I'm trying to help!
These guys have been fed at the same place and time every night for quite some time, so it will be a lot easier than if not. If you don't do that, maybe you should start. Then, what we will do tonight (will let you know if it works or not!
) is feed them all as usual, but try to keep the specific one away till everone else is done eating. She will still be hungry, so once the others are done, she should still be hanging around trying to get food. Then we will put up a trap (not sure what type, it's the trigger-type) with food in. And, if all goes well, voila! When we let her finally eat, the only food will be in the trap, so she will have to go in to eat it and it will go off.
If she's afraid of the trigger-trap, or if we can't keep her away from the food that everyone is eating, we will try a drop trap. With that, we can catch a few cats at a time, then you just transfer them as you want. The ones we don't want will go free, the one we do want will go in to the other trap.
Hope that helps! And hope some one else can give good advice, too, 'cause I'm sure there's more ideas out there.
Thanks for trying to help them!