This is the only really good thing about the game and it was enough to keep me playing it to the end. The mechanics are very, VERY thorough and well thought out and you can make the game as complicated or as simple as you like.
The basic idea is similar to ffxiii or others where you're a team fighting monsters and every monster has a 'field'. you need to chop through the field with enough damage/effort to cause a field break which creates a 2x+ damage situation. In some cases it's more like 10x - it'll feel like you're hitting the monster with a napkin until the field is down. There's a lot that surrounds this though:
- whenever a monster gets to his turn in the battle order, his field 100% resets
- the turn order can be manipulated - there are attacks that can delay a monster's turn, and monsters can do something called an 'order break' where they force themselves to have the next turn, resetting their field and attacking immediately
- the monsters, like your party, actually can have multiple turns within their turn (for your party, the number of turns you get to take at once is determined by your 'proficiency' aka level with your 'mic' aka weapon.
additionally, there are also bonuses that are slotted with a turn which is really tricky - if 1-5 represent your player and M represents the monster, you might see a turn order that looks like this 1 2 - - - 3 - 4 5 - - - M; the dashes are empty turns where a monster can order break or you can move yourself if you defend. some number of that list will have a modifier - regain HP, more susceptible to status effects, etc. So, for example, if you see that the monster will be in a turn slot that has HP recovery, you want to prioritize hitting him with a turn delay or two to get him out of that slot.
the usual slew of status effects / buffs / debuffs are in the game too, usually stacked from a single attack by a monster. there's also a disturbing amount of instant KO attacks at the later levels (at least on the higher difficulty) and weird effects that I was never quite prepared to deal with (there's a status that lowers the number of moves you get in a single turn I think? sometimes I suddenly had 1 or 2 moves instead of 4 or 5). and while not a focus, there's a lot of knockback attacks and (I'd never seen this) reverse attacks (pulls you forward instead). the idea is that you can line up cone attacks and other non-circular aoes with a knockback or reverse to a poorly laid out group but in practice it's virtually never necessary since you're swimming in very effective large circle shaped aoe attacks.
The balancing is jacked but I think by design - I was always surprised when a low level mob could wipe my party just by virtue of the bizarre balancing this game has. it seems like it's balanced for your team to perform 'harmonics' (team up mode where everyone takes their turn in sequence preventing order breaks and allowing for chain attacks that create new, sweeter attacks). there's a color scale identifying enemy threat - blue/green/yellow/red/purple/big skull - that describes how dangerous the enemy is in relation to you (there's no enemy levels that are visible). if you're prepared you can run in to a big skull enemy group of 10-12 with a surprise attack and wipe them all before they can attack with harmonics and the right chain skills (and you'll usually level up 3-5 levels immediately after). but if they get an attack in they'll wipe you out. conversely, you can be fighting a green (relatively safe) group and they can wipe out your HP in 1 or 2 turns if you're not paying attention. Bosses love instant KO or massive damage attacks. I don't know how many bosses there were where I had a max of 5k HP (and I think I was around the right level or above for the boss) and he would dish out 20k to two party members. There's also terrible options for SP management (the equivalent of MP in this game) - it's a constant complaint, but you'll find yourself out of SP after 3 or 4 fights and having to return to the office to replenish since getting more SP replenishing items is a hot pain in the ass.
Which brings me to crafting - why they decided to put even MORE crap in this game I don't know. But there's no store proper, you can create consumable items but it requires money (EP) and drops from mobs. So if you want to make (for example) a +25% HP thing, you'd need a small amount of EP and then you'd need to go hunt the mobs in whatever explorable area. all for the equivalent of a potion.
On top of all of the above, there's also outfits that are destructible (you basically have to keep two on hand to swap between since they break so easily even maxed out - if they're destroyed the slotted bonuses don't work in them anymore), lots of unlockables, affection between the boy support character and the idols to manage, the 'training room' dungeon which is a good portion of the games trophies (and out of 12 floors 8 of them are locked until post-game), and a lot more as far as the actual functions of the game.
I could keep rambling about this since it really does make the jrpgs I've played historically look really simplistic, but I'll stop there.
And the truth of the matter is I just spammed cosmic fan in harmonics and the big special attack each character has like I suspect most people do.