Antonyms of extremely:
meagerly, scantily, barely, hardly, just, marginally, minimally, scarcely, little, negligibly, nominally, slightly, somewhat.
Examples of usage:
  1. It was extremely mysterious.
    "The Mystery of 31 New Inn", R. Austin Freeman.
  2. She is, in my opinion, extremely silly, and a very naughty child.
    "The Rebel of the School", Mrs. L. T. Meade.
  3. " You are extremely foolish, Tom," said Harry.
    "Viking Boys", Jessie Margaret Edmondston Saxby.