Alright so here's how to win a bunch of chips in the casino in Ni No Kuni:
You might want to buy or win 5-10k or more chips before starting this.
I noticed tha tthe slow-down power-ups you can get in roulette stack, so what you want is to slow down the wheels X3. You can do this by using a combination of Lvl1 and Lvl2 slow downs items. Once you have slowed the wheels down x3 its pretty easy to line up 777 on the bottom row.
Doing so gives you a x100 payout on a 90 chip bet max and if you are in mega mode you can multiply all that by two. So a nice 18k chips right there... and you can do it two or three times in a row pretty easily.
To get into mega mode you have to fill up the seven flames at the top of the machine (over the prize payout list) and then line up and 8th one to activate it.
So if you have a X2 and X1 slow-down power-ups lined up you can wait till you get some flames (by lining up icons or using a flame power-up) and rack up the chips.
It can take a while to get a win since it is luck-based but if you have enough chips to start with, you are pretty much a guaranteed win eventually. The chests seem to randomly appear and give random power-ups so this can go pretty fast but you can expect to spend over half an hour to get a decent payout.
Just bet 10 chips until you have the required power-ups and raise your bet to 90 when you are in mega mode.
Or you could just play Platoon. Following @uniform 's tip, the dealer always seems to play her number cards on the right and her figure cards (king/bishop/wizard) on the left. So winning 6-7 games in a row is pretty easy. I made over one million chips by doing that once, by winning my prediction every round. So then you can trade in you chips for Medals of Agility and sell those for mad Gs.