Well Marlene isn't a fighter, she's powerful but in terms of offensive abilities... She's lacking.
However, she is classified by
Beiloune to be immortal and have complete protection from the dangers in his world and even Beiloun himself stated he can't kill her by normal means, making her durability that high but I think your actually asking about her "Overshadowed" nature.
Marlene is the first "Overshadowed" to have existed and do to being that way for so long she is above the normal Overshadowed. Going into detail, an Overshadowed being is someone who's shadows have become so thin the lose there place in reality and are no longer apart of history. Their bodies, minds, and souls are removed from the world and thus they cannot be touched, heared, or seen by anyone apart of reality. They are also capable of interacting with the world completely unnoticed, which can be explaind like this.
Example: Imagine sitting in an empty room with your friends with only one door and no windows and your all looking at the door. If an Overshadowed being walked opened the door and walked in, you wouldn't even register the door opening. Since they are no longer apart of the world/reality, their actions go completely unoticed as if they never existed and there actions never happed. So basically, you would need to be capable of seeing and touching beings who do not reside in reality. They aren't in another universe either, as they are forgotten and are not tied to any dimension or universe.
Marlene is stated by the other Overshadowed to be in a worst state then they are. All they can do is hear here but nothing more from what is shown. If Oracle can interact with beings like this then it's a stomp since I don't think Marlene's resistances are that good.
