using any of the 4 main actions (NOT run away, front, rear, or equipment) gives a certain amount of JP(job points)... the amount u get is lower after job level 14 (the amount differs by job class) but its 20 per action for the first 14 levels... so guard 4 times then attack and ull gain a job lvl. the lowest amount u can get is 8 JP therefore for that class it takes 13 actions.

its a sped up version of skill/magic system from ff2 (except the cancel trick doesnt work here)

hope this helps