Attacks with Sickle and Mythril Knife. Sometimes he'll steal your GP, too.
Final Fantasy VI Playstation One Bestiary entry
The Harvester is an enemy in Final Fantasy VI. As with most enemies in Zozo, they are dangerous enemies to face. On their second turn, Harvesters will use a Hi-Potion to heal themselves or allies. On their fourth turn, they throw Daggers and Mythril Knives at the party, which can be deadly. Their special attack, Sickle, is a slightly stronger physical attack. Locke can steal valuable Dragoon Boots from them, but they will retaliate by stealing gil from the party. Luckily, the party wins this gil back if they win the battle. The party should use their best attacks to dispatch the Harvester before they get their fourth turn. Bushido attacks, Tools, and Blitzes work well.
1st Turn: Attack (66%) or Sickle (33%)
2nd Turn: Use: Hi-Potion
3rd Turn: Attack (66%) or Sickle (33%)
4th Turn: Throw: Mythril Knife
If attacked by "Steal" or "Mug": Steal (66%) or Mug (33%)