For the most part, enemies are visible on the field instead of the traditional random encounters, which gives you the opportunity to either prepare for the battle or avoid it. Once in battle, you will control one character, the party leader, while any others in your party are on auto-pilot. You may not always have a full party, the max amount in any battle is three characters. Your characters' abilities in battle are determined by the role you've assigned to them, and you can change them dynamically during battle by switching paradigms. Depending on the situation you face, you'll need different abilities and strategies.
Before beginning to battle, you should decide on your fight strategy, then set up your party and paradigms to achieve it. For a large portion of the game your party will be set for you, but you will still want to train your characters in the Crystarium for the roles you need and that are best suited for them. Next, set up your paradigm deck to give yourself some options in battle. You'll need an offensive paradigm at the very least, and it's also good to have a defensive paradigm for healing when the going gets tough.
Besides your general strategy, be aware of your surroundings. Once you've encountered the enemies in an area, adjust your paradigms, equipment and party for best results. In even greater detail, you may want to make some changes based on the single upcoming encounter when you see an enemy. If you need them, shrouds can give you an advantage going into a fight, but they are rather rare, so use them sparingly.
In combat you will have three options available to you: abilities, techniques and items. Your abilities are based on the role you are currently in, and can only be changed by switching paradigms. Your first option is to use Auto-battle, which allows the AI to queue up your abilities for you, usually in the most efficient manner. For the overwhelming majority of the game, you'll want to let the AI control this part, as it is much faster and the AI takes into consideration the enemy's and your own strengths and weaknesses. You can also control your own abilities, in which case you'll need to queue them up yourself. If you have a sequence that you want to repeat, hit in the abilities option to load the last lineup.
Techniques, along with abilities, can be learned in the Crystarium. You can select from your known techniques during battle, but these all cost a certain amount of TP, which is not replenished after the battle is over, like your health. Your last option is to use an item. There are only a few items available in the game, but they can be useful as restoratives for your party, or for removing status effects or scanning enemies.
If you win the battle, you'll see the results screen and receive your battle rating, and you'll get any spoils. If you lose the battle, you'll see the game over screen, where you can either quit to the main menu or select "Retry", which will put you back in the field, right before you enter the battle. During the battle, you can press to go to the pause menu, where you can resume the battle or select "Retry" to go back to the field.
Ally status bars
In the lower right corner of the screen you'll find the HP bars for you and any allies in the battle with you. Once you gain access to TP, you TP gauge will be right below your HP bar, with a number on its right end showing how many full TP segments you have. In the middle of each HP bar, you'll see the three letter abbreviation of that character's role, so you always know who is doing what. Appended to the left side of your HP bars will be any status effects, either positive or negative. Positive status effects will be shown as blue icons, while negative ones will be red icons.
The ATB gauge represents the time that must elapse before your next turn, and limits how many actions you can take. It is shown in the bottom left-hand corner of the screen and is made up of multiple segments. When the battle starts or you finish your turn, the gauge will begin to fill up. When it is full, you will be able to select an action. Certain status effects and passive abilities can speed up or slow down how quickly your ATB gauge fills, allowing you to improve your attack speed if managed correctly.
Different abilities have different ATB costs, which are noted in your ability list. You can select as many abilities as your ATB gauge has room for, and you can earn more ATB segments through upgrades in the Crystarium. Techniques and items don't use up any ATB segments, but have their own costs for use. You can use any ability before the ATB gauge is full, provided that abilitiy's ATB segment is full. To start your queue before your gauge is completely full, press . Any unused time on your gauge when starting early will be automatically added to the gauge after your turn is over, and the gauge will start refilling from there.
Every enemy has a chain resistance and stagger point. Underneath the stagger bar in the top left corner of the screen, you will see two percentages in the form of "XXX.X% / XXX.X%". The first number is the percentage of damage you are inflicting on the enemy, and the second number is the percentage at which the enemy will be staggered. You start out at 100% damage, and successive attacks (including inflicting status ailments) increases the damage percentage. The chain resistance of the enemy determines how easy or hard it is to raise the percentage toward stagger. The stagger bar itself fills up with orange, showing how close the enemy is to their stagger point. When you reach the stagger percentage, the bar will be full. While your are attacking the enemy, the stagger bar fills up, and in between it slowly empties. If the bar drains completely, your damage percentage goes back down to 100% and you must begin refilling the stagger bar again. Although the bar empties when you are not attacking, if you perform another attack before it completely empties, the damage added to the bar starts at the highest point you had previously achieved.
When an enemy is staggered, the damage percentage climbs much more rapidly and maxes out at 999.9%. While staggered, the enemy's stagger bar drains down, unaffected by any attacks. Once the stagger bar has drained, the enemy will come out of stagger mode, and you can begin refilling the stagger bar again, starting at 100%. While in stagger mode, the enemy may also be more susceptible to certain attacks and status ailments or it might change it's attacks or defensive posture. If it's a strong enemy, you should check the enemy's info while it's staggered to see if you have any new options the help you against it. Certain Commando abilities, such as Launch are only used when an enemy is staggered as well.
Staggering enemies is the key to winning fights and winning them quickly. You approach to building up an enemy's stagger bar will depend on the circumstances of the fight. Ravagers are the best at filling stagger bars, while Commandos, Saboteurs and Sentinels stop the stagger bar from being depleted too quickly. Which paradigm you use will be based on the enemy's chain resistance and stagger point. For enemies with low chain resistances and stagger points, you can use a paradigm like Tri-Disaster (RAV RAV RAV) to quickly stagger the enemy without worrying about the bar depleting before you're done. For enemies with high chain resistances or stagger points, you'll want to throw in a Commando or Saboteur to make sure the bar doesn't drop to zero before the Ravagers have another chance to attack. In rare circumstances you'll find enemies that you either won't be able to stagger, or that can be killed faster with strong attacks. In this case, or when an enemy is already staggered, a paradigm like Cerberus (COM COM COM) can come in handy.
If a character has acquired their Eidolon and is the party leader, it can be summoned in the Techniques tab for three TP. When an Eidolon joins the battle, your other two party members leave and it is you and your Eidolon. Your HP will replenished and all status effects (good or bad) will be removed. Your Eidolon will attack or perform other abilities on its own, just like any other AI party member. Both you and your Eidolon can be attacked, although in the Eidolon's case, it doesn't have an HP gauge, but a timer until it disappears. The timer goes down over time, but it is accelerated when the Eidolon is attacked. Below the Eidolon's countdown timer is the Gestalt gauge. The more you attack and the better you do, the faster this gauge will fill up. The Gestalt gauge determines how long you will have when you switch to Gestalt mode.
Sometime before the Eidolon's timer runs out, press to enter Gestalt mode. In this mode, you ride your Eidolon and control its attacks. These attacks consist of the button in conjunction with a direction on , for a more powerful attack, or for a finishing move. You will see dial on the bottom left corner of the screen showing your options, how long the Eidolon has left and how much each attack costs. You can also press to use auto-Gestalt mode, in which your Eidolon will attack based on AI instructions.
After the Eidolon has left the battle, your entire party will be given full HP, all ATB gauges will be reset and all status effects on allies and enemies will be removed. This is an added bonus you should keep in mind, as it may make sense to use your Eidolon specifically for this boost rather doing actual damage.
One of the most important parts of being successful in battles is to learn your enemy's strengths and weaknesses. You can check to see what you know about any enemies in the battle by pressing . The first time you meet an enemy, all fields in your enemy intel will be question marks. As you fight the enemy, and more so over multiple battles, you'll begin to uncover enemy information, such as what types of damage it is susceptible to and which status effects you can inflict on it. Although you can eventually learn all this information through trial and error, meaning casting different elemental damage to find out what works, it's much faster to scan the enemy with the Libra technique or use the Librascope item. Once you know the information on an enemy, you and your allies will become much more effective. Buffing your weapon with an element your enemy is weak to can double the damage you do, and inflicting status effects can make the enemy weaker or unable to attack you. Your Synergists and Saboteurs can only be 100% effective if they have all the information.
Once you've beaten the enemies, you'll see the results screen and get your battle rating. The rating is based on how quickly you finished the fight compared to a target time. The target time is calculated based on your party's strength, including their stats and equipment. By gaining a preemptive attack, you get an automatic ×1.2 multiplier applied directly to the target time, making it much easier to come in under target. Your score is modified by how many seconds under or over the target time you are, with each second being worth a certain amount of points. You start out with 10,000 points, and your positive or negative bonus is added to this number to determine your final points. This final figure is then translated into your battle rating, based on a five star scale. You get the same amount ofno matter what your rating, but your spoils are based on how many stars you get. In general, the more stars you get, the better chance you will have of getting a rare drop, and the fewer stars you get the better your chance of getting a shroud.