In Pokemon, players expect a certain level of difficulty from the Gym Leaders and Elite 4 trainers, and as such, prepare accordingly. However, trainers can get caught off guard by getting forced into surprisingly tough random battles while traversing the overworld. This is because of teams that may not be fully healed, or because of a lack of options to deal with them at that point in the game.
These are some of the toughest random trainer battles that catch players by surprise due to their surprising difficulty.
Tough random Trainer battles in Pokemon
1) Bug Catcher Elijah in Kanto

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Elijah only has a Level 20 Butterfree when you fight him just before Vermillion. However, at that point in the game, it is fast enough to outspeed most opponents. While lacking any attacking moves, Butterfree can inflict multiple statuses on the opponent, including sleep (Sleep Powder), confusion (Supersonic), paralysis (Stun Spore), and poison (Poison Powder).
It is an even bigger threat in FireRed and LeafGreen, where it has the ability Compound Eyes. This ability boosts the accuracy of those inaccurate moves, making Butterfree even more of a pain to bring down.
2) Double Battle Thom and Kae in Johto

Thom and Kae are an optional double battle en route to the Safari Zone, using an Electabuzz and a Magmar. If the player has been fighting the relatively weak Pokemon in the game so far, the fact that these two have access to STAB Thunderbolt and Flamethrower is enough to catch most players off guard with the sheer amount of damage they deal.
3) Ace Trainer Jennifer in Hoenn

Jennifer may only have a single Pokemon, but it was enough to earn her a spot on the list. Her Milotic can seem like an insurmountable wall owing to a combination of its high Special Defense, its ability Marvel Scale, which gives it a Defense boost if statused, and access to Recover. Finally, the fight takes place on Route 120, where it is permanently raining, boosting her Water-type STAB moves.
4) Ace Trainers Dennis and Maya in Sinnoh

Dennis and Maya are another double battle the player has to face right before entering Veilstone City. What makes this fight hard is that this is the last fight of this route, so the player's Pokemon are most likely worn out and low on health and power points.
And then these two tag team the player with a lineup of Glameow, Monferno, Kadabra, and, the cherry on top, Gyarados! Unprepared players will wipe here and be sent back to Solaceon Town so that they can traverse all the way to battle these two again.
5) Backpacker Kumiko and Hiker Jared in Unova

Hiker Jared and Backpacker Kumiko are another double battle that players need to be wary of. While the player is teamed up with Bianca, who constantly heals your team before this fight, the two of them have four Pokemon that will have no issue wiping out most teams.
While the first two Pokemon they bring out, Golbat and Boldore, are minor annoyances at best, they are but the appetizers. The main course of this battle is a Darmanitan who knows Belly Drum to maximize its Attack, and an Excadrill who knows Earthquake. Even bulky Water-types available to the player at this point, like Jellicent and Alomomola, struggle to survive attacks from these two.
6) Poke Fan Family Jan and Erin in Kalos

Furfrou may not seem threatening, but when this double battle takes place, its stats can be hard to handle. Especially since this is yet another double battle, and both dogs have STAB 70 BP Headbutt, which has a chance to flinch. Considering that Furfrou has a Speed stat of 102 and nothing the player can catch at this point can outspeed them, expect to be flinched multiple times.
7) Teacher Emily in Alola

Teacher Emily is a good early indicator of the difficulty of the Alola games. In Sun/Moon, she has a Magnemite, a Steel type with multiple resistances, and an Alolan Meowth with STAB 60 BP Bite and the ability Technician, which boosts any move 60 BP or lower.
She gets even stronger in Ultra Sun/Ultra Moon, where she has only one Pokemon...the starter with an advantage over yours. As your starter is most likely your strongest mon at this point, this fight remains a challenge throughout all of Alola.
8) Colleagues Jordan and Alison in Galar

Colleagues Jordan and Alison once again are a difficult double battle as they lead off with a Hippowdon and an Excadrill. Hippowdon's ability, Sand Stream, sets up a sandstorm, which activates Excadrill's ability Sand Rush, doubling its speed. This overwhelming offence upfront can weaken teams for their duo of Steel types, Lucario and Perrserker, to clean up any remaining opponents.
9) Ramon the Student in Paldea

Pokemon Scarlet and Violet are the first games where players can skip a good number of NPCs in the overworld. Additionally, the odd level design of these games means the player might end up overleveled for even a few normally challenging fights.
But that doesn't mean the players will have an easy time if they decide to battle any of these NPCs for fun. Like Ramon the student and his Drednaw. It likes to set up with Rock Polish, boosting its speed by two stages and subsequently outspeeding most Pokemon at that point. This lets Drednaw potentially sweep if players aren't careful.
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