I tried a few 5* games - Classic Brazil vs Classic England.
The main difference in the opposition seemed to be how physical they were in tackling especially in midfield (picked up 2 injuries in the 1st half of one game), and then springing forward as soon as they've won possession.
They rarely broke through using passing moves, but took maximum advantage of my own mistakes, like sloppy passing in midfield.
I'd say controlled passing to keep possession is the first priority with opposition at that standard, otherwise the defence will dumped deep in it by their midfield team-mates so often, they're just run ragged.
I think the X + Square thing is to keep chipping away with X if your close enough (tackling), while the top of your thumb is leaning on the Square button (calling over a team mate). I tend to be printing with R1 at the same time.
Maybe it's the angle that matters most, where you try to nip in, and then steal away with the ball. Given time I try to avoid head on tackles (vulnerable to a side-step, and if you get passed your lost), or from behind (bookings). Instead I try dropping off the opponent, then nip in using X and L2.
(I still think L2 leans into the player and makes the challenge more physical, and then helps you shield the ball from tackles - but that's my impression, I don't think anyone else has a use for L2 apart from setting team Attack/Defence levels - ???)
Then try to escape with the ball still using L2, or switch to L1 or R2 to change direction.