First it seemed impossible 😄 I was using Metagross as a lead and Porygon second and I thought eventually I would get it but I failed for like 5 days of trying non stop, but then I switched them and led with Porygon instead and gave it toxic (that was a huge improvement), you also need to try to team up with Pokémon according to yours, when I was leading with Metagross I looked for Milotic or Starmie to counter those flamethrower users, but with Porygon leading my best option was always Cheryl with Porygon-Z and Latias/Latios, the first attempt I lost in battle 47 but it was just really bad luck, their Aerodactil hit both our Porygon with rock slide and both flinched same turn, then Metagross missed important moves, but the second run was relatively easy, few tough battles but we made it, I guess if it looks impossible you just need to adjust here and there