After you've shown the Max Honey to Mustard, he'll recognise your talents and allow you to fight him again, except this time he won't hold anything back. When you're ready, head to the Battle Court, via the back door of the dojo. Then speak to Mustard. Obviously make sure your Pokémon are ready to fight!
Dojo Master Mustard
|Urshifu (if you cleared Tower of Waters)||75||Fighting/Dark|
|Urshifu (if you cleared Tower of Darkness)||75||Fighting/Water|
It's likely that Mustard will be the strongest trainer you'll encounter so far in the whole of Sword and Shield. He's got a team of varied and generally powerful Pokémon, so you'll need to be on your guard.
First into the arena is Mienshao, an evolved Mienfoo. It likes to use Fake Out to cause a turn one flinch. It can also hit hard with Close Combat or pivot out with U-Turn. Flying, Psychic or Fairy-types are highly recommended. Ghosts could work as well, to nullify Fake Out and Close Combat.
Next, if he sends out Luxray, it's probably his weakest Pokémon, but still nothing to sniff at. This fully evolved Shinx has Intimidate to lower Attack; its moves include a powerful Wild Charge, plus Psychic Fangs and Play Rough. Ground-types are ideal. Special Attackers may fare well too.
Midday Lycanroc has a mean Stone Edge and can attack first with Accelerock. It can also set up Stealth Rock to damage switch-ins and surprise Fighting-types with Play Rough. Ground and Steel-types are your best bets here, as they take reduced damage from Stone Edge.
Corviknight plays more of a support role, setting up Light Screen to weaken your special attacks. But it can cause decent damage with the likes of Body Press, Iron Head and Brave Bird. Ideally, you want to hit it with Fire or Electric-type special attackers before Light Screen goes up.
Kommo-o is a dangerous foe that can use Clangorous Soul to boost all of its stats at the cost of a third of its HP. For attacks, it has Clanging Scales and Aura Sphere for STAB, plus Flash Cannon to deter fairies. Go with a Flying or Psychic-type. Or Fairy if they can withstand a hit.
For his final Pokémon, Mustard will send out Urshifu, but it will differ depending on which tower you conquered. If you cleared the Towers of Water, you'll face the Fighting/Dark Single-Strike Style Urshifu. Otherwise, it'll be the Fighting/Water Rapid-Strike Style Urshifu. Also, he'll Gigantamax it.
If Mustard's Urshifu is the Single-Strike Style, it can use Dark, Fighting, Poison and Steel-type moves. Fairy-types will absolutely melt it, but they'll also be hurt by Urshifu's Poison and Steel moves. If your Fairy is quite fragile, you might be better off using a Flying or Fighting-type.
Meanwhile if Mustard has sent out the Rapid-Strike Style Urshifu, it can use Water, Fighting, Poison and Steel-type moves. Against this slippery bear, you can safely choose from Electric, Flying or Psychic-types. Fairy and Grass can also work, but they're liable to receive super-effective damage.
Subdue Mustard and the curtains will close on your Isle of Armor story. As a reward, you'll receive Mustard's Rare League Card. Afterwards, you can rematch Mustard once a day by speaking to him in his room. Mustard will also drop a subtle hint about the future--this is likely referring to the upcoming Galarian Star Tournament.