from my experiences.. i reached out that:

for assistant skills, like freez, sleep, terra.. ecc, if u use them on a mob, then u make a big damage on it.. the skill will lvlup lots, like in burst: u use freeze, then fire uses Gs, then attacks, freez became lv 10..

for attacking skills, the mob must be must higher lv then you. thats all, and u gotta attack many times, more attack = more exp for skill.