You've got ideal weapons for the fight. I'm guessing you have Spectre Armor. A focus on dodging is nessacary for the fight.
When the summoning circle appears and the Cultist prepares to summon the Dragon, take some time without shooting to identify which is the real cultist and attack it, it's eye shape will be able to tell you the real cultist from the mirages.
If you have a UFO mount, that might help for dodging, you can also stay in the air with the Cultist. When it comes to aiming, it is trickier with the Lunatic Cultist as each time it moves, it cancels the lock on if you enable it. Using Aim & Use while using the right control stick to attack with your magic weapon will help for more accuracy. Learning the attack patterns, dodging them, and preventing it from summoning the Dragon and more mirages will help make the fight easier. Those are the best tips I have overall for the Lunatic Cultist.