It would have done the same amount of damage.
Multiscale (.5) * Critical Hit (2) = 1
Regular hit = 1
Also this argument has gone on for days, and the only thing we have discovered is something has changed or there is something we don't know.
Yes. There's definitely something that's been changed... The way I see it, the possibilities are as thus;
1: Rain's been nerfed and no longer powers up Water moves.
2: Critical hits have been nerfed, possibly due to the increased critical hit ratio from Pokémon Amie.
3: Hydro Pump's base power has been nerfed.
4: Sylveon breaks the mold in terms of Eeveelution base stat totals and has a Special Defense stat to make a Chansey jealous (highly unlikely, IMO, but just throwing it out there).
5: Sylveon was holding some new item that negates the effects of weather on moves aimed at it.