Here's the Unrivaled Empoleon battle where I soloed it with a Grumpig. This is one of the harder raid battles due to Empoleon's Hidden Ability, Competitive. With it, for every stat decrease it gets, its Special Attack goes up by one stage. Due to AI partners likely using stat lowering moves, Empoleon will quickly sweep you. You can get around this with Simple Beam, through the move is hard to obtain. Here's the setup:

Grumpig @ Shell Bell
252 HP/ 52 Special Attack / 204 Sp. Def
Nature: Calm
Ability: Thick Fat
Steel Tera
Flash Cannon
Nasty Plot
MudSlap
Simple Beam

Keep on resetting until none of your partners have Intimidate. Then, use Simple Beam as soon as possible. With that done, you can start setting up and attack with Flash Cannon. I used MudSlap four times so it would start missing for a good chunk of the battle, but it turns out I only needed to use it three times. This is because any stat change on a Pokemon with Simple is doubled, so each Mud Slap was lowering Empoleon's accuracy by two.

Then, I did 3 Nasty Plots so I could use Flash Cannon at maximum damage. It'll reset your stats at 80% HP and once again at 50% Time remaining, so make sure to use some more Nasty Plots. Once it erases its own stat debuffs at 50% HP, just keep on using Flash Cannon and you should easily defeat it thanks to Shell Bell constantly restoring your HP.