Ophie
Salingerian Phony
It also depends on the character. More so as time has gone on, they've tied a team's effectiveness to the character, namely how good they are at battling, what mood they're in, and how much they've grown if they're intended to develop.
For instance, Wally's postgame team in Omega Ruby and Alpha Sapphire is full of competitive single battle strategies. There's Talonflame with Gale Wings (a rare case of a computer player using Hidden Abilities prior to Generation VII), who uses Tailwind and Brave Bird. There's Magnezone who has Thunder Wave and other disabling moves. And the rest of the team, all of which are EV-trained. At this point, Wally has found the self-confidence he was looking for since you first meet him, and he learned a lot about Pokémon battling along the way. By contrast, there's Hop in Sword and Shield after Bede had mocked and insulted him but before he got his groove back in Circhester. At this point, Hop's team is all over the place, with every Pokémon swapped out but his starter, and he is a lot easier to beat than before or after. He's in an emotional slump, not having much motivation while still desperate to try to look good. In between is Nanu in the Alola games, who puts up a decent fight but becomes even stronger when he actually gains motivation after defeating him in his Kahuna battle.
The way I see it, if the character him- or herself hasn't changed much or at all since the last time you saw them, they're unlikely to be much easier or harder. Gains and drops in competence are tied to changes as a character. Or, in the case of folks like Hala and Milo, how much they choose to hold back based on how they perceive you, the player character, and your strength.
