Walkthrough: After obtaining the second flute and heading back to Poni Island, you must return to the Ancient Poni Path and head north (an area that was previous off limits). However, as you approach said entrance, you will see a number of Team Skull Grunts blocking the way. They are still upset at you for a number of reason, but mainly because Guzma is gone and all your previous attempts to stop them on other islands. You will be faced by one Team Skull Grunt to begin with, but after defeating him, all the Grunts will battle you as one in a Singles Battle.
Team Skull Grunt - Level 45 Ekans
Team Skull Grunts - Level 45 Fomantis, Level 45 Mareanie, Level 45 Houndour, Level 45 Golbat, Level 45 Raticate
After defeating all of them, Plumeria will appear from out of nowhere. She will begin by lashing at the grunts, primarily for being poor sports after all this time of grudge holding. She will then dismiss them, before apologizing to both you and Lillie for having kidnapped Lillie and Cosmog and for having brought all this trouble upon you and her family. She will then give you Poisonium Z.