I think Winona, though relatively easy in the game, has huge potential.
Her pokemon have the following type ranges:
Flying Steel Water Grass Dragon
Making her quite versatile, and you need a range of different attacks to take them down.
Additionally her pokemon know some powerful movies, for instance if you try and send out an electric pokemon to zap down most of her crew she can simply counter with Altaria using Earthquake. (With the exception of magnemite/magneton of course)
Also: Altaria - High Special Defence Skarmory - High Defence Tropius - High Health
So it takes some serious hits to bring them down.
They also know moves like Safeguard and Aerial Ace, making it harder to gain advantage through stat changing moves.
I would say Tate and Liza are hard to beat, because they have Psychic Pokemon, which are usually a pain to beat with anyone. If you try using Dark Pokemon, it will be tougher because are also ground, flying, and rock types. The Pokemon have high attack and that can cause a lot of damage to your Pokemon. Also the Pokemon on their side use moves that help out the other, since it is a double battle, which makes the gym tougher to beat.
Well you know that after beating the Elite 4, Gym leaders get new pokemons right? Well I think that Before beating the Elite 4 there's a tough Gym leader, and after beating it, another Gym Leader becomes the hardest to beat.
Before beating Elite 4: Wattson, Mauville Town Gym Leader, because at that time of the game you realy dont have any good Ground type pokemon with ground type moves, also his pokemons' levels are almost as high as your pokemons so you can't use that advantage unlike in many other Gyms, and they have some some realy tough moves, so everbody should be prepared when they go and fight Wattson.
After beating Elite 4: Well you know after beating the Elite 4, each Gym leader will get new pokemons each time, for 5 time, then their levels will not rise anymore and they wont change their pokemons. So I beleive the thoughest trainer then is Flannery, Lavaridge Town's Gym Leader, she'd have now 6 pokemons and they are: Arcanine Magcargo Houndoom Rapidash Camerupt Torkoal
I know it seems that having a good water or ground type pokemon would get rid of them, but not realy because their sp deffence and deffence is realy high, and Houndoom and Rapidash know Solarbeam which will kill your water or ground type easily since it has a high power, and most of those pokemons know Powerful moves like Overheat, Solabeam and Earthquake. So beating her should be a real challenge.
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