OK Shadow Ball definetely has its merits i was wrong.
252+ SpA Choice Specs Sylveon Shadow Ball vs. 0 HP / 4 SpD Jirachi: 254-300 (74.4 - 87.9%) -- guaranteed 2HKO after Stealth Rock
252+ SpA Choice Specs Sylveon Shadow Ball vs. 0 HP / 4 SpD Metagross: 278-328 (92.3 - 108.9%) -- 87.5% chance to OHKO after Stealth Rock
neither of them can reliably switch into Sylveon with rocks up and/or any prior damage
252+ SpA Choice Specs Sylveon Shadow Ball vs. 0 HP / 4 SpD Jirachi: 254-300 (74.4 - 87.9%) -- guaranteed 2HKO after Stealth Rock
252+ SpA Choice Specs Sylveon Shadow Ball vs. 0 HP / 4 SpD Metagross: 278-328 (92.3 - 108.9%) -- 87.5% chance to OHKO after Stealth Rock
neither of them can reliably switch into Sylveon with rocks up and/or any prior damage