After a certain amount of time has passed, or if Plant Brain takes a total of 533 damage, Blank will join the battle. Alternatively, Blank will arrive when Zidane is KOed or his health bar goes to a HP Critical state. Blank does not appear immediately if the player enters the battle with Zidane knocked out.
Plant Brain's most damaging attack is Thunder that hits the whole party.
Once Blank arrives, he and Zidane should attack using normal physical attacks. Vivi and Steiner should cast Fire and Fire Sword, respectively. Steiner, Zidane and Blank should beware of Plant Brain's Pollen ability as it can blind them and limit their physical attacking capabilities, although it will not hamper Steiner's Sword Magic.