Opposite words for systematic:
hit-or-miss, planless, disorderly, unsystematic, disorganized, immethodical, irregular, systemless, nonsystematic, haphazard, patternless, unordered, shambolic, chaotic, disordered.
Usage samples:
  1. This performance is peculiarly irritating to all systematic and ambitious parents.
    "Study of Child Life", Marion Foster Washburne.
  2. His education has been good enough up to now, I dare say, but it has not been systematic.
    "Mr. Marx's Secret", E. Phillips Oppenheim.
  3. Koreans bitterly complain of the ignoring of Korean history in the public schools, and the systematic efforts to destroy old sentiments.
    "Korea's Fight for Freedom", F.A. McKenzie.
  4. At first, when he formed a plan of systematic study, his object had been only the modest one of " general discipline," as he expressed it.
    "William Hickling Prescott", Harry Thurston Peck.