Shadow isn't an easy game. I still haven't beat the Final Battle, closest I've come is a handful of shots left. I top out around 1.5 billion, but a typical "good" game is probably only 900 million or so. Thing is, I can have a 30 million game right after that, and after that, it's definitely not the kind of game most people can just pwn at will.
A good part of it is how you're doing with your scene start shot, it's tight, and if you're not able to hit it pretty reliably it's tough to get too deep. But that's part of the beauty of the game, not only is it challenging, you can play it in different ways. Skip the scenes all together and try and rack up super vengeances instead!