everybody knows AMU's mesprit's effect to make the retreat of all AMU stuff EL ZIPPO!! if I have an Ariados in play, is the retreat 1, or 0?? same if moonlight is in play, is the retreat 1 or 0??
Any time there is a card that reduces a Pokemons retreat to 0 makes anything that increases the retreat cost of that Pokemon not matter. They could have 3 Ariados in play, and you have something that gives you 0 retreat and you will still have 0 retreat.
Phoebe's stadium reduces retreat by 2, if your pokemon has a retreat of 2, then you play Phoebes, they have no retreat, but can be affected by Ariados.
Now if they had fluffy berry, their retreat is now 0, meaning ariados cannot affect it.