I think the leveling works like this. Every skill can level up and contributes to your next level, however, the higher the skill = the more it contributes to next level. So going from destruction level 50 to 51 gives more progress compared to going from level 24 lockpick to level 25. You can train destruction by hitting people with destruction spells . Also I'm level 21 Nord that uses magic, one hand sword and sometimes a bow.