Opposite words for mild:
consuming, violent, ungentle, overcast, breezy, irritating, roughened, abrading, screaming, heavy-handed, thick, hard, bleak, aggravated, misty, rude, intensified, windy, deep, brutal, stern, caustic, gloomy, wild, severe, harsh, stiff, hazy, trigger-happy, stormy, keen, blustery, tyrannical, utmost, intense, bitter, fierce, roughening, extreme, raging, uttermost, dirty, big, gruff, cloudy, rough, rugged, smart, unabated, gray, foul, savage, terrible, exquisite, foggy, forcible, grim, tearing, scathing, powerful, blustering, gusty, vehement, main, blood-and-guts, inclement, strong, unrelenting, overwhelming, raw, abrasive, terrific, showery, intemperate, sunless, proud, forceful, rainy, coarse, intensive, wicked, profound, dismal, pitiless, cold, bad, concentrated, squally, oppressive, nasty.
Usage samples:
  1. It was the month of March, and they were just driven out; the weather was mild, and the grass had appeared, but the wind was bitterly cold in the evening, and it began to rain.
    "Anecdotes of the Habits and Instinct of Animals", R. Lee.
  2. Henry Ward was a mild specimen compared to this one!
    "The Pillars of the House, V1", Charlotte M. Yonge.
  3. There used to be much turf talk, and sometimes a little card- playing and mild gambling- but Barty's tastes did not lie that way.
    "The Martian", George Du Maurier.
  4. The Astronomer's mild voice rose to an unusual pitch and he said, I will speak.
    "Youth", Isaac Asimov.