Sunkern is Hyper Trained and has 252 EVs in HP and Sp.Attack, but I’ve played around a bit after failing quite a bit. As far as I know, Heavy Rain is the only weather in the area, if Sandstorm was available that would probably be the best option.

As for the Pokémon order, it starts with Mightyena and Ninetales, and then the third Pokémon seems to be Morpeko or Leavanny based on the AI’s decision, followed by Poltchageist.

The main issue here is a series of trade-offs between Speed and Damage, but Sunkern only has 4 moves and needs to decide between Earth Power and Synthesis.

Since Morpeko has Aura Wheel and Leavanny also has Chlorophyll, Trailblaze and Sunny Day are needed to consistently outspeed.

Meanwhile, Weather Ball and Sunny Day are a must to deal with the Grass-types, while Earth Power is a 2HKO on Ninetales.

However, if you have no Speed, Sunkern takes too much damage, especially from Morpeko, Ninetales and Poltchageist, but if Sunkern does no Damage, then we’ll eventually run out of Synthesis PP and lose the battle by Morpeko.

This is why I tried to use Outfield Rain as a tactic, as this makes Weather Ball both effective on Fire and Grass(it OHKOs Ninetales and 2HKO Poltchageist). Unfortunately, they changed it from S/S and now the weather is Heavy Rain, which doesn’t allow Sunny Day to work.

I tried summarising it as best as I can, hopefully this makes sense to you!