Tested this on DSWoW, it may work other places as well.
Well, you need to be in a FFA area. When a player of your own faction flags up FFA all as well, engage them in combat. Now depending on where you are, the other member will most likely try to unflag FFA if they're almost dead. If you stay targeted to them even if they unflag themselves you can continue to kill them.
I'm not sure if this has been posted before so feel free to delete it if ya want. I'll post a ss later if this is new :P