Offense.

Some weapon skills improve more easily than others.
For example, firing bursts or cheap hits.
After killing, the killer gains more experience.
Critical Strike skill progress is ambiguous.
With the double trouble, it is up very fast.

Defense.

The Evasion is very difficult growed instead the Armor.
Only the one who takes the hits can improve the Armor.

I have to expose the character to bullets in order to improve evasion and armor skills.