Oh man.
I just did this, and it's not easy. Timing, skill, and luck all play vital roles in getting this.
In order to "beat" adventure mode, you have to clear stage 13-1: Space. If you get to this stage after completing stages 12-1 through 12-7, you CANNOT play this stage on its own if you fail. You have to beat those levels ALL OVER AGAIN.
As for stage 13 itself, it's not that tough once you figure out the pattern (check out video), but the level itself does take significantly longer to complete than any of the other levels (I can confirm level 12-1 takes 5000 points to beat, IE new balls stop entering the screen). Coins provide an 800 point boost, but if you can get a high chain bonus (always shooting and scoring) early on, it does make a difference.
Gaps are tricky here. Do them if you have a wide enough opening, but don't stress them, as you would in the other levels. Momentum is key here. Time your shots to line up perfectly, because if you mess up too many (even just a few), you're done. Hell, my hands were shaking before I got this.
If you start from level 12-1 and DON'T lose any lives, you should in theory, have 4 lives (you should be able to get 100,000 points with ease, with an extra life coming every 50,000) at the bare minimum. Personally, I always screw up on 12-5, so I lost two there, and lost one on 13-1.
Remember the powerups. Given the level is very clustered, a bomb power-up can make a collasal difference if you're in a hot spot.
Other than all that, luck plays a pretty large role here, in terms of ball placement. Early on, I got tons of same colored balls next to each other, and got up to a 13X chain or something like that. Once you beat the level (NOT clear all the balls yet), it seems that it's smooth sailing from there. I didn't find this at all challenging.
Like I said at the beginning though, luck, skill, and timing play HUGE parts in this. This is NOT an easy task (having a 5.0 ratio just for beating the game), but with perseverance, you can get it eventually.