Easiest way to achieve this is to keep the Freelancer job on one of your characters. There will be certain instances when you are forced to change. Mage is best when you have to fight while miniaturized and Dragoon is best for Saronia. Other than the few times you are forced to change some characters use the Freelancer job. You should get it before your playthrough is done. I had all 4 characters Freelance job at 99 before defeating Odin.
Now to understand Jobs a bit more you will need to know that a Job increases a level for every 5 turns that character has in that job. That means the character has to perform a job action. That means choosing rear\front. items, fleeing, missed turns due to ailments or enemy dying before that character can make an attack do not count. If the character is dead at the end of the battle they do not get credit either. You can also only gain 1 lvl per battle. That means fighting someone for 10 rounds still only gains your characters 1 lvl.
If you are looking to train a job level, the best way is to go to the beginning area of the game and fight the weaker enemies. Use Guard or any other job related actions like Steal or Study ect for 5 turns and then kill the enemy. If you match your classes right, for example, Viking is a slow class so I know when I was training my job levels, my theif and scholars would always go first. So I could attack the final enemy on round 5 with the viking and avoid having to attack on round 6 because of not knowing who would go last.