The idea here is to concentrate fire on the guy floating in the centre (Ki) whilst periodically taking out the cultist minions that drop from the cage, and dodging the fire balls. You can shoot Ki anywhere and as displayed in the video below courtesy of 'max18363' he will eventually die.
Once Ki is dead you can use Jones to possess the poor crucified b*stards above the revolving iron maiden and use the
button to open it for a short time. This allows you to shoot and kill the one in the iron maiden.