Steiner will always use the attack that causes him to explode into oglops on Blank, even if Blank is unconscious and thus normally untargetable.
The player commands Zidane, Garnet, Vivi, and Marcus. Steiner is accompanied by Haagen and Weimar at first, but they will flee when their HP is depleted. The HP of both is low, so defeating them should be first task. Steiner has nothing to steal this time, so the player can focus on offense. Steiner will never attack Garnet, and with her Cure spell, there is no danger of defeat.
During the third battle, a Bomb grows behind Steiner. The party attacks him and the battle ends with the Bomb exploding. The party is the same as the second battle, but Steiner is alone and again has nothing to steal, so the player should just attack until the Bomb detonates.
Alternatively, players can just defend and wait due to the battle being timed.
With the use of Gameshark codes on PlayStation, or cheats on the PlayStation 4, Steam, and mobile versions, players can inflict over 100,000 damage to Steiner before the Bomb explodes.