Alolan Vulpix only gives one prize... In my mind using AV to set up Metagross, then sacrificing up to two Metagross to Blacephalon is enough to win the game. Sacrificing AV will cost them 1-2 energy. Each Blacephalon uses 5 energy to kill a Metagross. The last Metagross will survive the last attack that Blacephalon is able to use. Any Naganadels that try to fight will get OHKOed, along with a Max Potion to remove the damage.
This all assumes, of course, that the opponent does not GX and weakness is not a factor. If the opponent does GX, then you can only afford to lose one Metagross. Ultimately, this is Metagross's worst matchup, although it is possible to win, though unlikely. Newgaleo will fair much better.
As someone who mains Metagross/Solgaleo and usually destroys the opponent, this makes me very sad.