if you have a rocket launcher, and RPG, just go to a safehouse (can't remember what the actual name for them is called) go outside, kill everyone around the place, while still getting easy access back to the safehouse, go in, and warp to another entry point... do the same and repeat.
That'll get your explosive skill up easily, and it'll reload your ammo every time you warp.
Same goes for shooting, but it takes longer, because you can't really kill multiple enemies.
For driving, just try doing the races, that's your best bet... or go to the car place, can't remember where it is though... (there's a spiral-y ramp, and a long jump ramp)
and just keep going over those until you level up.
The easiest way to get your strength up is to go on tough mode, and just punch everyone. And you can't get above 4 stars,
when you get the bar up (so it should go to 5 star) the nuber place will turn to a "--" and the bar won't go any higher, and won't fall when you have to regenerate. you get an achievement for making all stats get to (--)
Here he sits on his lonesome again,
a common sight to see,
this prospect-less man,
cold and alone,
begging his soul to be free,