Enhancing is a way to get rid of units that you don’t want and make space for some new ones, but if you are running really low on Gil, you can always sell units too. The higher the level and the higher the rarity of the unit, the more Gil you will get in exchange for selling it, so if you need some new equipment, but don’t want to go through the battles or search through the exploration areas, then sell some characters instead.
You will get some special characters who are absolutely useless for fighting, but are especially effective for earning experience and Gil. For example, the Metal Cactaur is useless in battle, but if you use it as a sacrifice when you enhance a character, it will give that character 10,000 EXP in one shot. If you sell a Gil Tortoise, you earn 10,000 Gil in one single shot.
Not so far into the game you will come across the Farplane for the first time. Once you go there once, you will be able to access it from the towns – the first Farplane warp will be in the town of Mitra. Wander around the Farplane and talk to the googles, and when you can, come back and talk to King Mog. You can also get into the dimensional vortex here.
The dimensional vortex consists of several dungeons that will unlock for a limited amount of time, either a specific number of times per day or for a specific time span. Many of the dungeons will feature huge levels of experience points, Gil, or materials for crafting. Check back often to see which new ones have opened up, and check your notifications to see which ones will be open at any given time.
Later on in the game you will unlock the Colosseum. You can access it after you unlock Royal Capital Grandshelt. You’ll fight multiple rounds of battle, just like the old-school Final Fantasy VI colosseum, and the farther that you go, the more points you earn, and rewards that you gain. Get far enough and you will fight a boss battle. Win the boss battle for even huger prizes.
Later on in the game, you will unlock the ability to equip your character with Espers. An Esper will give stat boosts to your character, imbue them with an element (which has its own strengths and weaknesses), and new magic spells, among other goodies. Esters can gain levels and get stronger, as well.