Ni no Kuni: Wrath of the White Witch

After you finish the storyline at Tombstone Trail, the casino will open. It's to the right, right as you enter the dungeon, and you can't miss it. Once you clear the game, they will open a special VIP room section, which you'll need to buy tickets (S rank prizes) for in order to get in. You'll need to buy all 4 tickets (costs 75,000 coins in total) for the trophy. You can either wait until you have the 500k guilders trophy and just buy the required amount of coins (total cost 375,000G at 5G per coin, and you can trade 5,000G at a time), or you can attempt to get them yourself via gambling. If you do plan to gamble, the best way to do it is to play the slots. You'll need about 3-5k coins to start. Start playing at 10 coins per spin, and keep going until you have time slow items that add up to level 3 or higher (lvl 1 x 1 + lvl 2 x1, or lvl 3 x1, or whatnot), and an item that gets you into bonus mode. Set it to play for 90 coins per spin, activate everything, and aim to get a 777 jackpot on the slots. With level 3 time slow, this will be very easy, and will net you 18,000 coins. You should be able to get at least 2 jackpots per set of items.

Sidenote: The A rank prize items will unlock later in the story.