Antonyms of mild:
intense, deep, intensive, keen, wicked, blood-and-guts, violent, thick, strong, profound, wild, overwhelming, proud, smart, tearing, trigger-happy, concentrated, consuming, uttermost, big, unrelenting, inclement, main, raging, terrible, extreme, exquisite, severe, utmost, aggravated, bad, unabated, brutal, screaming, intensified, terrific, vehement, fierce, cold, blustering, blustery, breezy, gusty, rough, squally, stormy, windy, misty, rainy, showery, bleak, cloudy, dismal, foggy, gloomy, gray, hazy, overcast, sunless, bitter, dirty, foul, nasty, raw, harsh, intemperate, powerful, forceful, forcible, savage, roughened, rugged, abrading, irritating, roughening, grim, gruff, rude, stiff, heavy-handed, oppressive, pitiless, tyrannical, abrasive, caustic, coarse, hard, scathing, stern, ungentle.
Examples of usage:
  1. " Kind is a mild way of putting it," remarked Leonard.
    "Only One Love, or Who Was the Heir", Charles Garvice.
  2. Unfortunately, the minister Perdriau, a mild and an amiable man, took it into his head to tell me the members were rejoiced at the thoughts of hearing me speak in the little assembly.
    "The Confessions of J. J. Rousseau, Book VIII.", Jean Jacques Rousseau.
  3. Another and mild form of Dharna is that known as Khatpati.
    "The Tribes and Castes of the Central Provinces of India Volume II", R. V. Russell.