Opposite words for extremely:
slightly, scantily, minimally, somewhat, just, little, hardly, meagerly, marginally, scarcely, negligibly, barely, nominally.
Usage samples:
  1. I have just thought of an extremely amusing way of doing it.
    "The Admirable Crichton", J. M. Barrie.
  2. Extremely beautiful woman, and that gold thing looks well on her dark hair."
    "Queen Sheba's Ring", H. Rider Haggard.
  3. For a man his size, he was really extremely quick.
    "Legacy", James H Schmitz.
  4. The time was when I might have believed extremely well of myself, but that day has gone by.
    "Dave Darrin's Fourth Year at Annapolis", H. Irving Hancock.