Best Place To Level Up?

If you say you can take it but not dish enough out, I suggest the sunken submarine. Bring Yuffie w/ her ultimate weapon and Morph. (SPOILER)Every single enemy down there can be turned into one Source or another for permanent stat boosting. And the enemies give off decent EXP/AP/Gil too. Though I advise equipping Ribbons (Morph Master Tonberry if you need an extra).

Seph is a wuss; anyone with decent defensive skills (yourself included) can take whatever he can dish out and come back swinging. However, his (relatively low) 100,000+HP can be a problem if your team isn't well-leveled. I recommend getting your characters to L99, equipping 4x-Cut on Cloud w/ Ultima Weapon, and hacking away for 40,000 a turn. Get Mime for one or two other characters, say to hell with switching parties, and keep on slashin'. (SPOILER)Or just come into the battle packing a fully-charged Omnislash, press the button, sit back and enjoy the show.

I would really suggest the Mideel forest... When you fight headhunters, they give you reasonably nice amounts of exp and ap. But yeah, you can't beat the movers for ap or the Magic pots on both exp and ap. Simply redicurous.