An easy skill to max out is the Strength. Use Commander Jaxson (Terry Crews) as your agent for the 10% boost for your strength.
As you progress through the story, you'll eventually get the opportunity to become a high alert target and initiate lockdowns by performing a lot of carnage. The easiest way I found to initiate them was to find a roaming convoy and destroy it. Most of the time this would put my agent on alert.
Once the alert or lockdown begins, just melee kill everything they throw at you and you will rack up the xp. I'm not sure if difficulty matters, but I did this on agent.
I went from 5.50 to 6, which is when getting xp really begins to grind, in less than an half hour. So doing this, on top of the strength you already accumulated, will help you max out that skill fast.