Oh boy.
Well, the ending to mission 8 isn't too hard. Mission 9 should be pretty challenging for you - there are two fun shootouts. But you'll eventually get it.
I just beat it... man, what a great game. Truly the best game I've played on any console this year, and I've played a lot of good ones. I'm going to re-start it soon and try and explore everything. God it's great.