Diacorns are pretty simple creatures. They'll either physically attack you, or they'll use Ground to inflict some Earth elemental damage. It will only take one or two hits from Seth or Kaim, depending on your level, at this point in the game to kill them. Having Jansen hit them with a Wind spell will also work, even if he has yet to level up a single time.
Weak Against Wind
Encountered- Ipsilon Mountains