It's not hard. Do some special tricks to raise your base score and then increase your multiplier and you can drop 1mil+ in the first 30 seconds.

Here's one of the top players (prevzzy) dropping 1 bil on Cruise Ship. He gets to around 8 million by the 1:30 mark:


https://www.youtube.com/watch?v=Wk3Y3Hqyj_A

Extremely easily.
You build up your base score with special grabs, then you build up your multiplier with manuals and grinds. When you're grinding or manualing you can press different combinations of the face buttons to quickly shift into different tricks.

https://youtu.be/7CXgm8S94Wc?feature=shared&t=117
Here is tutorial on special tricks
getting 10-20 mio combo is fairly doable in the game, everything over it is actually pretty crazy x)

are your stats maxed? because i can't balance for doodoo even with max stats on manuals and rails

Over 20 years of playing the games.