Persona Q uses a random encounter system, like many older RPGs and even the Etrian Odyssey games. During the exploration of dungeons, players will notice a little icon in the bottom right corner of the upper screen, which signals how close to fighting a random counter the player will be. At first, it starts out as blue and will turn green as you continue exploring, eventually becoming yellow and then red, which means that a battle will occur very soon.
The battle system is turn-based, similar to the Persona and Etrian Odyssey games, borrowing mechanics from both. There is an ambush system in place, where either you or the enemies can get a full turn before the other can perform any actions. At the beginning of any turn, you input the commands of that character, but the actions aren’t performed right away and are instead performed according to the AGI stat of them and the enemies.
The One More System is a battle system that is a part of the Persona series of games, where characters would get extra actions by exploiting the weaknesses of foes. Persona Q uses a variation of that system, with it being called Boost. Instead of gaining an extra turn, whenever you hit a weakness or get a critical hit, you will enter Boost state. You will know a character’s in Boost by their portrait being golden in battle.
What Boost does is enables that character to go first on the next turn, ahead of everyone and everything else (with some exceptions). It also makes all of their skills cost 0 HP or SP, letting them basically take a free action. The catch is that if a character takes damage between attaining Boost status and their next turn, they will lose Boost. It also goes without saying that not getting another critical hit or hitting the same/another weakness will also result in losing Boost.