Nice photos, thanks for sharing!
I agree that you should try to do posed shots. During my first cosplay event I just took photos of cosplayers who just happened to be posing.
When you ask cosplayers for a photo, they will pose for you and most of the time they would look at you even if there are other people joining in to take photos. Also, you can ask them to stand somewhere nearby where there are less background distractions.
If you're going to use lightroom and shoot raw, that will cover up a lot of exposure mistakes, but not focus, so nailing the focus is the most important thing to pay attention to.