Possible spoilers if you haven't gotten to the end of the game, I guess.
Hm, when I beat the Elite Four I had seen 144 in the Sinoh Dex. That left two "normal" and four "legendary" Pokemon. I had forgotten to visit Iron Island, and a trainer there had one Pokemon I needed (The one who's leaves cover it at night or in a cave; I forget its name) and the team-up partner gave an egg for the other I had missed.
Then, three of the "legendary" Pokemon were the lake ones, and finally Palkia, which you can obtain the "seen" status elsewhere.
Fighting every trainer I see is something I frequently do in Pokemon games, and I'm also a big fan of "One really high level Pokemon, and several lower level fellows" style of playing. Despite NPCs always saying that's not the way to go, that's how I roll.
I think Empoleon was 72 when I got to the Elite Four, and my next highest was a Medicham at 47 I caught in Victory Road, followed closely by a Graveler, 46, also from Victory Road and a Staraptor who was the third Pokemon I obtained, at 45. I had little trouble with the Elite Four, though the Champion did almost kill me by whoring Earthquake.
Haven't played too much since. Opened up Pal Park and traded my unfinished team from Leaf Green. Did some battle towering, but I'm to understand there's a lot more to do after you get the National Dex, so I'll have to look into it.
Anyways, if anyone needs any sort of Pokemon, and I have access to it, feel free to ask. I'm more than happy to send out eggs or whatnot.