Every character has the same basic options shown in the above picture. Self-explanatory ones aside, the two interesting options here are
Focus and
Change. The former charges a character's mana which is very important because your characters are going to have around a 100 MP, and your spells cost anywhere from 20 MP to 50 MP on average. The attack option is generally worthless and pretty much all of your characters are going to use spells for their damage output, so evidently recharging is very important. Although the total amount of MP each character grows pretty fast as they level up(which also influences how much they recover each time they focus), your spells' mana costs never go down. It's also fun to note that HP recharges after each battle but not MP, and there's no MP restoration consumable in this game as far as I know.
Change is the other weird option that in my opinion, really makes this game shine. Any character can immediately swap themselves or another party member currently on the field (you can only have 4 at a time) with one of the other 8 reserve members. That's where the 12-member team thing came from. You'll have two tanks and two DPS out, and suddenly find the need to quickly switch one out for a debuffer to slow down the enemy's ATB gauge(think like old FF). And then once the boss's special attack is ready you switch out your non-tanks entirely so only the tough ones get hit. And you switch your tanks out when a boss is about to inflict the party with poison so there's less people to heal, but that has it's own danger because if your front line is defeated it's game over. What I'm saying is it allows for a lot of complexity in battle.
Other things that make this interesting imo is the battle formation. As you can see it's a 4-party straight line, but the placement actually does matter. The leftmost spot is the one much more likely to be targeted by enemies, and the rightmost is the opposite, meaning that's where you'll want to place your frail characters. Also, aside from single-target and party-wide attacks, there are row-attacks which is basically AoE but the amount of damage diminishes for each succeeding character to the right. The leftmost character takes the most damage and it decreases until the rightmost character who takes a small fraction of the damage. The same applies to enemies and naturally you have access to row attacks as well, so it allows for some interesting party customization in order to build the most effective setup for quickly farming mobs and for battling bosses.
Interestingly there's an evasion stat, but it does absolutely nothing because attacks don't miss. They used to be able to miss, but the devs patched it out entirely and somehow the game still functions very well. They left the ability to boost it in which is pretty shoddy of them, but just know to not bother with it.
With all this and how many characters are available to you I hope I've sufficiently demonstrated how much variation and depth there can be with this combat system against fun boss fights. I was fighting Youmu and lost a bunch of times, and finally won after I figured out the right setup and strategy, and didn't have to grind at all.
