The battle system in Final Fantasy XV is action-based and takes place right where you run into enemies on the map. Sometimes enemies will show up right on top of you, initiating combat, but most of the time (especially on the overworld), enemies are already on the map. When you approach their vicinity, you will see a red line appear at the top of the screen, which indicates that the enemies have noticed you. You can either sit and wait for them to approach (has one advantage), or step out of the red outline on your minimap to escape from the enemies.
Once you get in close enough, you can start attacking enemies by holding down the Circle/B button, which will swing your currently selected weapon. You can switch weapons by pressing the corresponding direction on the D-Pad, and you can even switch out those weapons mid-battle via the main menu. Each weapon has their own moveset attached to them, usually accessible by pressing a direction with the left analog stick along with the attack button, so experiment and see what works best in the given situation.
One of the biggest things you will need to manage during battles is your health, which is divided into two aspects, normal HP and max HP. The majority of the enemies you encounter during the day will cause damage to your normal HP and if this falls to zero, then you will be put into Danger status. When this happens, your max HP will gradually deplete and should this fall to zero, then you will die. There is a few seconds after dying in which you can use an item to revive Noctis. Enemies at night, or daemons as they are called, will cause damage to both your normal and max HP values. A lot of items cure normal HP, but only a few heal max HP, such as Elixirs, Hi-Elixirs, Megalixirs and even magic with the Healcast property.
One aspect of the battle system that was added in the Day One patch was something called Wait Mode. Those familiar with some of the previous games in the series will know that this basically lets you choose actions without the action playing out, so you can take your time without having to worry about your characters dying. The same general idea applies to this game's version, but Wait Mode is activated by simply standing still in combat.
While standing still, time will stop and a meter will appear at the bottom of the screen, which will deplete very slowly on its own while time is stopped. Once it's depleted all the way, battle will resume as normal. One of the biggest advantages to using Wait Mode is whenever you lock-on to an enemy, you can hold down that button and a meter will begin filling up around that enemy. When it's full, you will have effectively scanned the enemy and their health and resistances will appear in the upper left.
The only problem with Wait Mode is that scanning enemies only applies to that particular one and only lasts for that battle. Other enemies, even those of the same kind, need to be scanned separately and on a battle-to-battle basis. Also, this information isn't stored anywhere, so unless you have a very good memory or write it down, you might forget an enemy's resistances. A resistance being orange means a weakness, purple means the enemy is strong and a black with arrows means the enemy nulls that or absorbs it (very few enemies have the latter).
With some of the bases down, it's time to delve into the more complex aspects of the battle system. One of the basic abilities that Noctis has is the ability to warp using the Triangle/Y. While locked on to an enemy, doing this will allow Noctis to perform a Warpstrike on the enemy. He also has a dodge roll with Square/X, but warping can also be used to avoid attacks, especially those that cannot be phased through with Square/X.
You will also notice certain points are highlighted with a warp opportunity. These are Warp-Points and you need to hold the button down when one's in view to warp to them. Sometimes, these are on top of rocks, but others are on the sides of cliffs. If Noctis is hanging there with his sword, then your stamina is being depleted while hanging, so you'll need to watch out for that. The main idea being using Warp-Points is that when you do, your MP is fully restored, plus your HP is regenerated as well (not Max HP).
Blindside and Linkstrikes
While you're fighting enemies, you will undoubtedly notice that you will sometimes do link attacks with your friends or even do more damage. Blindside attacks work exactly as they sound, as hitting an enemy in the back will deliver 1.5 times the normal damage. Also, with certain weapons equipped (swords, greatswords and polearms), doing a Blindside attack will sometimes result in a Blindside Link, where one of your friends will join in for a joint attack, dealing a lot more damage than usual.
There are other forms of Linkstrikes in the game, one of which can occur after you block an enemy's attack and follow up with a parry. Some enemies have attacks that will prompt a Square/X icon to appear on your screen. Hold this down and not only will Noctis block the attack, but you can quickly follow up with Circle/B to do a parry. Who you will Linkstrike with is random and there is a slight cooldown between them, so you can't just repeatedly perform them.
On the left side of the battle screen, there is a green bar that will gradually fill up over time in battles. This is the Technique bar and once it reaches certain thresholds, you will be able to hold down L1/LB and then issue a command to one of your allies to perform a technique. Depending on the technique, it will take from one to three of the bars to perform and each one has a different effect.
All of your allies can equip new techniques by first purchasing them from the Ascension Grid (under the Techniques nexus) and going into their equipment screen. They all start with one equipped at the beginning of the game and can learn up to four more (Prompto can learn a fifth and Ignis learns another automatically as part of the main story). As you use each technique in battle, it will level up and you might see a "Critical" stat on them when you go to equip them on an ally.
This critical stat will determine the opportunity for a character to use an enhanced version of their technique. The normal versions will have a green background when the name appears, but enhanced versions have a gold background. Consuming a character's favorite meal will guarantee they will use their enhanced version. The differences between both versions can mean more hits or a longer duration of a buff. As an example, Gladio's Tempest normally does two hits, but if he breaks out the enhanced version, it does three hits.