It was Sapphire version. I had just beaten Winona of Fortree City and got my Feather Badge (after a long battle itself since her team was quite annoying if you're starter is a Blaziken, however she's a Gym Leader so I expected that). I continued on my way onto Route 120, going through trainers without much difficulty... UNTIL HER! Ace Trainer Jennifer. A
boss in mook clothing, as TvTropes calls it (and checking their Pokemon page for that trope, she's mentioned there and has quite a lengthy entry there!). So what's so difficult about her that she almost wiped my team? She has a Milotic, the first one in the game you encounter (unless you caught a Feebas and did your hardest to evolve it). I had a Blaziken but I fought other Water-types before, all I got to do was use my strongest attacks. Eats it, just ate my attack and for an entree had my Blaziken by downing it with a Water-type attack (forgot which one). My only other Pokemon able to "take on" this monster was Wobbuffet by Spirit Bonding it, but yeah after the battle I ran back to the Pokemon Center and decided to train a bit on that route so that wouldn't happen again.
But I got my revenge in ORAS. in Alpha Sapphire my Starter wasn't a Blaziken, it was a Sceptile. That I could Mega Evolve. I did my strongest Grass-type move and watched its HP drop from full to zero with satisfaction. Sapphire me have been vindicated.