Setzer may well be my favourite Final Fantasy character from any game, but I've never liked his special ability. I decided to take a little look at it today and it turns out the game actually cheats. So it's not just that I've always been bad at using it... Even if you're good with timing there's only a small chance you'll pull it off!
I found an interesting guide on gamefaqs about it which is worth a read if you like seeing the silly ways abilities are coded in these games. It turns out there's a counter which gets incremented each time an action goes off. It gets incremented by a fixed amount, so you can know what the counter is currently at. Then the value of the counter determines which slot results are legal. So 8 of the 256 options allow the 777 auto-kill to go off. If you're not futzing with the counter and it actually is in a 'random' spot then you're only able to get the best outcome 1 in 32 no matter how good you are at spinning the wheels.
Also, apparently you can cheat at spinning the wheels by pausing and unpausing the game to see where the wheels are.
Apparently the RNG starts each fight at 0 and there's a trick to get it up to a 777 auto-kill value in one action. Use an echo screen and then go to slot town!
I donno. I used to think I was just bad at using slots. Now that I know it's a deliberate design decision to make the ability bad I'm not very happy. I like when things are hard because they're hard. I don't like it when things are impossible because of game design. At any rate, I no longer have any desire to try to use the ability. I am glad I know how it works so I can avoid using it. I guess all Setzer will do now is be awesome by throwing dice at the enemies!