>>38089913A Gardevoir with max HP and Defence will only take at most 62,5% of its max health from Poison Jab (no item boost since Phero needs Focus Sash) which is enough for Gardevoir to survive both a Feint coming on the second turn, as well as Poison damage.
Gardevoir's Moonblast will bring Phero down to at most 10% of health even if Phero has max investment in HP and SpDef and Gardevoir has no SpA investment. Since Pheromosa is down to easy KO range, and only has access to Feint as a priority move which can't KO Gardevoir, Gardevoir can easily nab the KO with Shadow Sneak.
Even if Pheromosa tries to go for Z-Poison Jab, Gardevoir can hold a Focus Sash (since it doesn't need an item for anything else) to survive and KO Pheromosa since opting for Z-move requires giving up Focus Sash.
The only way Pheromosa can win is if it gets a crit on the first Poison Jab.
