Most of the game is full of easy battles favoring the use of glass cannons because they win battles the quickest. When you get to the elite 4 it suddenly becomes challenging. It really catches you off guard. The glass cannons get slaughtered.
The same happened to me in the original version, maybe they didn't change the balancing.
I had to grind so much and with no trainers left for easy experience points I ordered a cheat device because I didn't want to waste my time.
Most of the game is full of easy battles favoring the use of glass cannons because they win battles the quickest. When you get to the elite 4 it suddenly becomes challenging. It really catches you off guard. The glass cannons get slaughtered.
This is just how Pokemon games used to be. Gym leaders were slightly more challenging than they are nowadays but for the Elite 4 you actually had to stock up on a lot of potions and revives, revives and the occasional full restore for during battle and potions for topping everyone up between battles.. Alternatively you could grind but the E4 is the most efficient way to grind.