So we need to keep trying until hit the highest tier to get the bonus? It seems that it is really hard, because I have only hit lower tiers and when I do the double I never win once before

. Logicaly think that you might never achieve higher tier because if you keep on doubling you will lose in the end. So I do not know how much do I need to spend just to get 1 bonus point
No, you don't need to reach the highest tier to get the bonus points!If you reach the highest tier doubling up, you will receive your money.
But if you "lose" while doubling up, you will receive bonus points!
The conversion rate is 1 Bonus Point per every $0.20 (*if the cash winnings aren't fully divisible by 0.2, there is a 50-50 chance to convert it to either 1 or 0 bonus points).
Let's say you are playing with $1 and win $2.
You decide to double up and you win, now you have $4.
You decide to double up again, and you lose.
These $4 will be converted into bonus points.
So you would receive: ($4 0.20) = 20 Bonus Points.
Then you can use these bonus points in the Super Slots to win bigger prizes!
Statistically, it's better to always double up your winnings in the Mini Slots until you get the bonus points, and then use them in the Super Slots, there you have better chances of winning a bigger sum of money!
I hope it's clear now, if you have any other doubt just ask
