The Justice Alloy was eaten up by a Pileated Snagret. Olimar must throw Purples at it to kill it and avoid when it goes underground. Once it pops up, he must keep the Purples away from it, but he shouldn't underestimate his Sprays. Olimar must get the Nectar x his Purples have leaves.
Another strategy is to use White Pikmin. It should eat the Whites and the boss will be poisoned.
To get more sprays, he should keep on getting various berries. It is an important idea to get more as he'll run out of sprays.