When I was going for these over the summer, for the last two I just camped out at the spawn point and waited till it showed up, jumping in on events as they came up. If you let people know that you need a particular one, people are usually good about calling out the spawn in map chat. I had the best success on the weekends. When you're waiting for that last one, it can seem like forever though. :) One thing I heard or read, don't know if it's true, that you have to keep the NPC leader at the spawn points alive when they get swarmed or else the legendary doesn't spawn. I made sure to jump in and fight, not just sit there.