Here are a few broken mechanics off the top of my head.  I actually have a muuuuch lengthier and detailed breakdown of the borked parts of the game, but this'll have to do for now. 

1 - You have absolutely no control over your movements in battle.  this wouldn't bother me except for the fact that many battles rely on movement-placement strategies and being in the wrong place at the wrong time can and will hurt you.  You have no control, therefore no ability to avoid this fate should the AI chose it. 

2 - The Paradigm shift animations resulted in exposure to enemy attack with no ability to defend or even prepare a counter.   This wouldn't  be an issue if not for the fact that the enemy ATB bars continue to fill and they continue to attack you when your bars are frozen and are incapable of doing any action or moving.  Simply having the battle stop for that split second while you change paradigms would have fixed this, but instead it's a broken mechanic that most certainly should have been fixed. 

3 - Too much reliance on random chance.  Much of the summon/eidolon battles (it's been a while, I don't remember what they were called) required that you fulfil a certain task to complete.  my friend would do it with ease, and complete it first try.  I would do the exact same thing and fail a dozen times.  sometimes the bar would fill faster than others in spite of our stats, move lists, and abilities all being identical.  

1. Except when it does Whilst certain actions provoke the boss, you still have no control over the formation or arrangement of your party so rather than spreading out when you want them to they'll position themselves in all manner of formations, sometimes bunching up (relatively) so that area/splash damage attacks hit multiple party members. This forces you to shift to a healing paradigm and puts you on a back foot. In previous games I found the odd boss fight frustrating whereas I was fequently frustrated with this aspect of the game/battle system.

2. And this is one of those fights I found frustrating, lol. I'm not sure if you can go back but I think you can get the items you need in the earlier fights. I stopped my 3rd playthrough because I forgot how annoying this fight was and ended up getting killed for healing my party too well (wtf?!).  Slightly off topic but I personally thought that the battle system in X was one of the best battle systems in Final Fantasy (ignoring the horribly frustrating battle just mentioned). Slower than XIII but tactically sound.

Here are a few broken mechanics off the top of my head.  I actually have a muuuuch lengthier and detailed breakdown of the borked parts of the game, but this'll have to do for now. 

1 - You have absolutely no control over your movements in battle.  this wouldn't bother me except for the fact that many battles rely on movement-placement strategies and being in the wrong place at the wrong time can and will hurt you.  You have no control, therefore no ability to avoid this fate should the AI chose it. 

2 - The Paradigm shift animations resulted in exposure to enemy attack with no ability to defend or even prepare a counter.   This wouldn't  be an issue if not for the fact that the enemy ATB bars continue to fill and they continue to attack you when your bars are frozen and are incapable of doing any action or moving.  Simply having the battle stop for that split second while you change paradigms would have fixed this, but instead it's a broken mechanic that most certainly should have been fixed. 

3 - Too much reliance on random chance.  Much of the summon/eidolon battles (it's been a while, I don't remember what they were called) required that you fulfil a certain task to complete.  my friend would do it with ease, and complete it first try.  I would do the exact same thing and fail a dozen times.  sometimes the bar would fill faster than others in spite of our stats, move lists, and abilities all being identical.  

4 - Random weapon/accessory upgrade system.  I understand that there's apparently a method to its madness, but after 60 hours of playing this game, as well as looking at countless faq's and testing for myself, I saw no patterns and what worked one time wouldn't work another time.   Too much reliance on random chance and too little explanation to give you a hint. 

5 - Unavoidable instant kill attacks.  I know there are items to prevent these attacks somewhere in the game, but I didn't get them.  your ability to survive should NOT be entirely reliant on chance (did you find the item?  did you equip it? did you even know it was an issue?).  the final boss had a move that would kill one of your members in one hit.  It was 100% effective unless you had a certain item equipped and had a 1/3 chance of hitting your leader, resulting in a game over.  it wouldn't be so bad except he waits until I think his second or third stage (after 15-20 minutes of battle) to do it, then he does it almost constantly and of all the FAQ's I read, I found no defense against it that I had at my disposal.  

6 - To make it worse, a lot of enemies had attacks like this.  the game literally demands you either be clairvoyant or persistent (keep playing and dying to learn the boss's patterns).  Many bosses and even some regular enemies would change stances or attack relentlessly without any foreseeable warning, often instantly killing you.  Yeah, sometimes they give a warning, such as flashing lights or a stance change, but you have to know exactly when something's about to happen and how to counter it or you will die, and reacting often isn't enough due to the fact that attack patterns change so quickly you're dead before your paradigm shift animation is complete.  This happens entirely too honest, and unless you're psychic and fast, you will die.  a lot. 

7 - Above all else, you die a lot.  In fact one could argue that a game mechanic is trial and error by death.  there's no way a reasonable person could beat 90% of this game without dozens of needless deaths, and that is a flawed mechanic in an RPG.  sure, if you wanna reserve these instakill moves for hidden bosses or hunts, then by all means go wild.  Some of the world's best hidden secrets had to be found during trial and error, but when almost every battle has a combination of random chance and trial and error like that, it's a flaw. This could have easily been fixed by tweaking the damage the enemies did or how much health you had or even making it so you had more warning or you didn't instantly get game over when your one character died.  

A lot of these flaws in the combat system could be fixed easily.  a tweak here, a few lines of programming there.  Instead we have a battle system that doesn't actually work.  I mean sure, people can beat the game,  but you shouldn't need to do the virtual equivalent of shooting in the dark over and over again waiting for random chance to not fuck you up.  as it stands the battle system is broken, and since it would require so little to fix it and make it more balanced, I'm convinced they developers did it this way intentionally, so that angers me.  

To me, a battle system like this would be like playing fallout three and there was say a 1 in 100 chance a bullet would backfire and instantly kill you, or like if you were playing mario and some mystery blocks exploded, sending you back to the beginning of the level. 

Oh no, kill me now, every point you made was completely game breaking and definitely not present in other Final Fantasy's.

...Your obsessive hating is really starting to become a nuisance now, you're grasping at straws too.

"You die a lot"...Seriously? I thought most of you FFXIII haters insisted that FFXIII was too easy? So what if you die a lot, battles aren't trial and error, it's one of the only battle systems in a game I have played where victory or loss boils down to skill, not level, unlike all the other Final Fantasy's I have played. And also, you are making death in FFXIII a bigger deal than it actually is. If I die in Final Fantasy XIII, the only time I lose is the time I spent on that battle, 20 minutes max on very rare occasions when a boss kills you at the last moment.

As a comparison.

In Final Fantasy VIII, I grinded through Ultimecia's castle but couldn't find the last monster so I could unlock the save ability. I fought all of Ultimecia's forms and died on her last when she had 5,000 hp left, losing 2 and a half hours of progress due to an overpowered and lucky hit and the fact she stole all of restoration magic.

Why don't you call Final Fantasy VIII "the worst game evaaar!" then? Please tell me, which battle system is ACTUALLY broken?

If I remember correctly, in FFVII, Sephiroth casts a spell in his second form which brings all your party members' HP to 1. If you don't heal quickly enough, you die and have to start over from your last save. It could be a fairly big progress you lost depending on where you placed the save point.

FFVIII's battle system is broken because you can repeatedly summon without any penalties. It's much more effective summoning than doing regular attacks. I rarely drew magic, because summoning was much better. After a while, i got bored doing the same thing. That, along with a horrible story and cast (imo), is the reason why I stopped playing FFVIII.

1. Position has almost no effect on the outcome of the battle, unless, like you said, there are powerful attacks over a large area. This only became and issue with some of the summon battles, still, paradigms effect your positions, switching somebody to a sentinel or commando will send them to the front and take all the damage, when you switch back to another paradigm, everybody is spaced out. It's still something hardly worth mentioning.

2. I can't find any holy water in the drops though, this is beginning to get irritating, I'm only playing for the story, I might as well give up and watch the ending on Youtube. And I'm not too keen on FFX's battle system, it relies too much on items and chance in my opinion. Some characters are far more overpowered than others too (especially summons). I would say my favourite has to be FFVII, it's simple and it works, FFVIII was also good and I enjoyed the junctioning system, it's just when you use that magic, your stats go down.

1. We're going to have to agree to disagree as I thought this was the most flawed part of the battle system. I'd much rather have it where either 1 person got hit or the entire party got hit so I knew what I was in for.

2. Don't give up! You're actually quite near the end, although the lack of Holy Water is annoying. Have you tried stealing off the enemies rather than just looking for drops? I'm pretty sure theres a way to stock up on Holy Water before the fight. Also, I'm not sure why you're using so many items unless you're using Rikku a lot. For most statuses I use Esuna which I believe you get fairly early. If you use the alt sphere grid you can even get multiple characters learn Esuna. I actually quite liked the fact that each of the characters felt different and had their own strengths and weaknesses (except for Kimahri who's just crap, lol) and the fact that you could just switch them in and out of battle added a new dimension to the play. It's all quite tactical knowing who to use and when rather than having all the characters the same.

VII & VIII were fun and simple, but in VII it felt like all the characters were the same except Aeris/Aerith meaning the tactics for everyone were very similar, and VIII I felt made everything too easy. Once you realised the Junction system could boost your stats I never used magic (with the exception of Aura) or GFs as my characters were all doing max damage and once you had the limits through Aura, boss fights were simple.