I think Renzokuken is better than Duel.. I like Duel, don't get me wrong, but as good as Renzokuken? Come on now! Hehe.. I guess it's just a matter of personal preference.
Degenerator is nice but so damn cheap. I used that once to level up Eden to level 100. I made Quistis equip Eden, put her HP in the yellows, gave her the ability to always start off every battle and I think there was something that made her snatch all experience as well, or something? Not sure. In any case, Degenerator at the start of every battle and Eden was level 100 fairly quickly
I don't like Eden too much though, it wastes too damn much time (I just wanted to see it's maximum damage). Or any other GF for that matter, as should be obvious from my post above :P
And leveling up is severly overrated in FF8. That's the worst thing about the game.. that they ruined the whole point in leveling up. Apart from that, the game is actually quite fun, but far from the best FF game