Crackdown 2

Tips for boosting skills:

Agility will be maxed out by the time you find all Agility Orbs.

Strength can be easily maxed by using ground pounds in large groups of freaks or Cell.

Explosives will likely be maxed for most people simply from doing freak breaches. If not, Shrapnel grenades cover a huge area and easily kill almost any type of Freak or Cell.

Driving will be maxed by the time you complete every Road Race and Stunt Ring. If you don't want to do either, constantly doing tricks like powerslides and J-turns will provide small boosts. If you don't want to do that, the easiest method is to plow into huge groups of Freaks as every kill will count toward Driving skill.

Firearms is probably the toughest to max. Firearms includes any weapon that is not an explosive, including mounted turrets and vehicle guns. Go to Unity Heights where there are tons of Freaks at night, and drive around with a buggy firing into the crowds. As you kill more Freaks, bigger ones will show up which give huge boosts to Firearms skill. During the day, beacon areas underground will usually have tons of Freaks in them if you haven't detonated it yet.

Even after finding every orb in the game, my Firearms skill was still only at level 5 with 15 toward level 6, but a few solid "nights" of game time killing Freaks maxed it out.