My first game was Sapphire. I went through the entire thing with just my starter, a Treecko/Grovyle/Sceptile. It hit Lv85 by the time that I reached Ever Grande City. My next highest level was just a Taillow/Swellow that I used for Double Battles. It was Lv24 by the time that I reached Ever Grande City. My next highest was a Zigzagoon at Lv5. Yeah.
Not bad enough? Well, here was my Sceptile's moveset: Strength, Cut, Rock Smash, and Flash.
Yeah, it was a LEVEL 85 HM SLAVE. I didn't realize that HM Moves couldn't be forgotten, nor did I realize that a Move Deleter was in the game so that I could forget them.
But wait, it gets better.
The second member of the Elite Four, Phoebe, uses Ghost-types, which are immune to Strength, Cut, and Rock Smash. Additionally, Sableye is on her team, which is immune to Flash as well, thanks to Keen Eye as an ability.
I actually still have yet to beat her, and I refuse to erase my save file or actually put in the effort to delete these HM Moves and Heart Scale back some good stuff, just because it was my first save file, and I feel sentimentally attached to it. (Actually, even without changing movesets around, I suppose that I could just Flash them until their accuracy is minimal, then switch out, she misses a bunch of times, and I slowly kill her with Swellow (who is immune to Ghost-type moves to boot). Huh. I'll keep this in mind.)
Oh yeah, and there's another big facepalm moment in this playthrough.
So take a wild guess what I used my Master Ball on in this playthrough. I didn't realize that there was only one in the game, so I just wasted it on a Cacnea. Fortunately my battery died not long after, though, and so I got another chance to not waste it.
And so I caught a Golbat with it.