Even though it seemed like Adam was having an easy time dealing with Fenrir at the moment, the reality couldn't be farther from what it looked from the outside.
Was it really easy to infiltrate the Sea of Subconscience of someone else?
The answer was that— it really depended on the person in question. So, whose mind infiltrated was a big factor in answering that question.
The variable here was the strength of the resistance that the person's mind showed.
The current resistance that he was going up against to invade Natasha's mind was extraordinarily immense. Just existing and projecting his consciousness inside this place was extremely difficult for him and was causing him to lose a lot of energy.