Lance McDonald, a prominent figure in the FromSoftware fanbase, spoke with Bandai Namco staff, who confirmed that the DLC will be more challenging than the base game.
“The DLC is aimed at experienced players and they are willing to make it more difficult to push the limits of accessibility for new players.”
It is clear that this is a general statement. Elden Ring has very difficult enemies such as Malenia and Two of the Divine Skin, so it's hard to believe that Shadow of the Erdtree will have more hectic and difficult fights.
The developers are hinting that fans should brush up on their skills and practice before the expansion's release, if FromSoftware really sticks to this approach.
Mesmer the Impaler looks like an opponent that is capable of multiple compilations of rage. And judging by the expansive map, regular enemies may pose a greater threat than the base game's enemies.