Garchomp is probably the worst Pokemon to be weak to in Pokemon Brilliant Diamond and Shining Pearl.
Players can have all the type coverage and strong Pokemon they want; if they don’t beat Cynthia, they don’t become Sinnoh League Champion. Without a doubt, Cynthia’s largest threat is her Garchomp, so trainers really shouldn’t even step on Victory Road without a counter to it.
What Pokemon can trainers use to beat Cynthia's Garchomp?
Garchomp is both Dragon-type and Ground-type. This means it’s weak to Fairy, Ice and opposing Dragons.
Here’s the bad news, though: Cynthia’s Garchomp has counters to all of these types. It has a Yache Berry (cuts the power of Ice-type moves), Poison Jab (hits Fairies for super effective damage), and Dragon Claw (hits Dragons for super effective damage).
As a result, trainers need to be very strategic about how they handle this. It may even take two Pokemon to eliminate Garchomp.
Ice-types are still effective at defeating Garchomp, who is 4x weak to Ice moves. The issue is getting rid of the Yache Berry. There are two ways trainers can do this: by making Garchomp lose the berry, or simply having the berry activate and hitting Garchomp again.
Unfortunately, most of the Ice-types in Pokemon Brilliant Diamond and Shining Pearl are rather frail, so there’s no guarantee that they will survive an attack from Garchomp. There are two exceptions to this, though: Cloyster and Mamoswine.
Cloyster is likely the quickest way to deal with Garchomp. This is because of Icicle Spear, which hits 2-5 times. Yache Berry will activate on the first hit, but afterwards, all the other Icicle Spears will do super effective damage.
If trainers really want to just annihilate Garchomp as quickly as possible, they may want to consider looking for a Cloyster with Skill Link. This ability ensures that any multi-hit move will hit the maximum amount of turns. Therefore, Icicle Spear will be guaranteed to hit five times, and no Garchomp is surviving that.
Mamoswine can deal with Garchomp with the move Ice Shard. It’s only 40 base power, but this move is always guaranteed to go first. So what trainers can do is take a hit from Garchomp, make it burn it’s Yache Berry with a different Ice move (Avalanche, Ice Fang, etc), and then Ice Shard it for the knockout.
Without these Pokemon, trainers can also get rid of Garchomp’s berry first. This would be through using moves like Knock Out or Bug Bite. Afterwards, they can go into their Ice Pokemon and likely one hit KO Garchomp.
Outside of Ice-types, there are a couple other ways trainers can get past Garchomp. One way is to use a Fairy with Kebia Berry. This item cuts the damage from a super effective Poison move in half. Therefore, it would ensure that a Pokemon like Gardevoir can survive a Poison Jab and then Moonblast afterwards.
One other slightly cheesy way trainers can solve the Garchomp problem is with Levitate Bronzong, since it resists one of Garchomp’s attacks (Dragon Claw) and is immune to the other two (Earthquake and Poison Jab). From there, it can either Toxic stall or Calm Mind to oblivion.