It was really depressing for me. It took 4 days for me to get unlock.In Zone 4, The Casino, you will unlock a door upon defeating 29 monsters.
When that door opens, you have access to slot machines.
Stand under any slot machine and pressto activate the machine. Pressagain to spin it.
It costs $10 a spin. The $10,000 prize is for lining up 3 sevens.
This trophy is the only real grind in the game.
It could go very fast for you, or it could go very slow. It seems to just depend on luck.