Holding the body and turning the outer cover can be hard if it's very tight. It might be easier to do if the fingers used to hold the main body were in a rubber glove or had finger cots, that way they wouldn't slip. They can feel really tight, especially if put on too tight before.
This is a shot of the joint where it comes apart when it's tight.
This is a shot of the joint with the cover part way off.
This shows the cover and battery removed.