I had trouble with it too at first, but this is the system I got into:
Get really, really good as one or two characters (I chose Luffy and Sanji). Then play the game on Very Hard mode, get as any coins as you can (for some reason, missions worth more coins tend to appear on Very Hard), and then when you're at the slot machine, blow them all. As far as I can tell, it is impossible to get into Bonus Mode in the slot machine by sheer skill, so just keep blowing coins until the entire slot machine goes black except for the Jump Pirates. Then aim a little bit to get three of those in a row (the slots will do most of the work for you), and then you go to another slot machine. Repeat the process above until those slots go dark, then aim for the ?s, and so on. It will take a LOOOOOONG time before those ?s will even get you characters, but once you get your first your second should follow quickly (I got all three of my characters in one trip to the slots with 90 coins).
It's really repetitive, so just keep trying with a bunch of different characters so you don't go insane.
EDIT: Oh, and a note: make sure you're really good as those characters. It took me half an hour of straight trys to beat the final boss on Very Hard as Luffy, but only five minutes as Sanji.