As for weapons, the best advice i can give is to watch the training videos. I didn't realize exactly how targeting system worked, and then one day i was bored, and decided to watch them. Now, with a little patience, i can pretty much take out anyone with a headshot using only one or two rounds every time.
I think its so pointless to drive when you can jump from building to building. Plz send me a friend request to help me in driving. Why Bother19
I've also pretty much maxed out everything but driving. The best way i can find to get more driving exp (besides running people over), is to just do road-races or find stunt markers to drive through. Stunt markers are usually hard to get at, but yield a big exp boost. If you've got an opening in a game, send me a FR/invite. I'm usually just wandering around by myself, trying to find those last 20 agility orbs, or trying to boost my driving.
gt: toowicked4u
EDIT: I was actually able to max out my driving skill this weekend to complete the Master Agent and Timed Stunt Driver achievments. Many thanks to Why Bother19 for pointing out all the jumps on the beach in Los Muertos.