It's possible for a level-one Rattata to defeat much stronger Pokémon, thanks to something known as the F.E.A.R. strategy. When combined, a particular mixture of Pokémon moves and an item let the F.E.A.R. strategy beat even high-level Pokémon.
The F.E.A.R. strategy is named after the most popular configuration of the technique: F for Focus Sash, E for Endeavor, A for Quick Attack, and R for Rattata. A Rattata, in particular, isn't actually required for this technique, though, as a number of low-level Pokémon can do the job. It's just important the Pokémon can learn both Endeavor and one attack with a high priority, like Quick Attack or Swift. It's also possible to breed a Pokémon with these moves.
Since the Focus Sash functions in a similar manner to Sturdy, the Pokémon holding this item will survive any attack from full health with a single hit point, if it would have been knocked out by that attack. Endeavor is a Normal-type move that forces the enemy's HP to match that of the user, and Quick Attack (and other high-priority Pokémon moves) will activate before a slower move. Taken together, these elements have an obvious and deadly combination.
The Power Of Pokémon's F.E.A.R. Technique
The basic F.E.A.R. setup involves breeding and hatching a level-one Rattata that knows Endeavor and Quick Attack. Once it has a Focus Sash equipped, it's ready to roll. The next time it enters battle, it would likely be killed by the enemy's attack. The Focus Sash will allow it to survive with one HP, and it can then use Endeavor to force the opponent's hit points to match its own, dropping them down to one. In the next turn, Rattata can use Quick Attack to defeat the opponent, as it will deal at least one point of damage, regardless of level.
The F.E.A.R. strategy isn't foolproof. For one thing, it doesn't work on Ghost-type Pokémon, as they're immune to Normal-type moves, unless the F.E.A.R. user has the Scrappy Ability. It's also possible to kill the Rattata with a trap or weather effect before it can fire off Quick Attack. The strategy is far riskier in battles with multiple Pokémon, too. The F.E.A.R. technique is more of an interesting concept than something that should be feared - apart from when it's used by an NPC trainer in Sword and Shield, but even then, they ruin it by only using a single Pokémon.