Not only that, but her ace, Dragalge, is incredibly bulky specially, and most Psychic types attack using special moves. and while they do for the most part, they usually run into trouble with her Drapion, which can ignore Psychic-type attacks and KO them with super-effective moves. Since the most common Ground-type moves are now out of the question, you might think that Psychic-type Pokémon will work better. You fight her in a double battle, and the field for the battle is the Wasteland Field, which weakens the moves Earthquake, Bulldoze, and Magnitude (which, as they hit all Pokémon in battle, would have made the fight much easier) to a fourth of their typical damage it also boosts her Pokémon's Sludge Wave (which hits every Pokémon except the user) and Gunk Shot attacks.
Oh, and don't even think about trying to terminate the field effect, because as of Episode 15 doing so just reverts it back to the Corrosive Field. It also still boosts the power of his moves. This field, rather than deal damage upon every switch, poisons every non-Poison/Steel Pokémon you have at the end of each turn. It gets even worse on the story path where you out him as a member of Team Meteor in this path, he floods the room with gas, turning the field into a Corrosive Mist Field.
To top it off, his ace, Crobat, has a massive speed stat and can raise its special attack with Nasty Plot, giving it the power to fire off obscenely powerful moves (including the aforementioned Venoshock) left and right if it gets half a chance. It doesn't affect his Pokémon, though in fact, it actually boosts the power of some of his moves, such as Venoshock (essentially giving him a move with 130 base power and no drawbacks in your third gym battle). His field effect, the Corrosive Field, damages every Pokémon that switches in with type-scaling Poison-type damage (so it's basically a Poison-type Stealth Rock that can't be removed). It gets even worse in Pokémon Reborn Hardcore, in which ZEL gets a Jolteon (which pretty much always moves first at this point in the game) and Taka gets his Gligar and Klefki from his first battle with you. ZEL is no slouch, either they have a Glaceon (which is very bulky and can hit very, very hard), an Espeon (which is rather frail but is very fast and powerful), and an Umbreon (which is bulky as heck and has tons of status-inducing moves).
If this wasn't bad enough, most of their Pokémon are a pain: while Taka's Lileep (Exeggcute in later versions) is pretty easy to handle, his PULSE Tangrowth hits like a truck and has incredible defenses, while his Chatot has great speed and can wipe entire teams if it sets up a Nasty Plot. To start with, you have to fight them in a double battle.