This particular challenge can be quite tough when you first have access to it. It may be best to return when you have access to Devil Trigger.
For this Secret Mission, you need to defeat seven enemies (Five Stygians and two Ravagers) within the time limit. The tough part is that you can only damage enemies while they are inside of the green circles that keep appearing and disappearing in the area. Over time, these areas will shrink down to nothing and you'll have to move to a new area to continue fighting.
Your best friend is probably going to be the Ophion Demon Pull. You can pull enemies into the green areas or, better yet, pull them into the air above these areas which allows you to focus and possibly kill enemies without being attacked by the ever persistent Ravagers.
