I love Fate/Extra's system but uh lemme try to get my notes
Basic Stats
STR - Increase physical damage to enemy
DEF - Decrease physical damage from enemy
AGI - Stabilize damage both from/to enemy, increase Skill speed
MAG - Increase magical damage to enemy and decrease magical damage from enemy
LUK- Increase Critical rate and increase the chance of not getting negative status.
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The Battle System
(Clockwise)
Attack defeats Break
Break defeats Block
Block defeats Attack
The game is more of a glorified Rock Paper Scissors mechanic. Battle scenes consists of you choosing which mode of attack to use Attack, Guard, and Break. Attack wins Break, Break wins Guard, and Guard wins Attack. Only that you've got to do it six times in a row.
Each round of battle consists of 6 turns. You generally don't get to see what your opponent is going to do, so it's like a game of rock-paper-scissors with commands that you choose in advance. Certain active/passive skills and status ailments may reveal the enemy's turns.
The standard commands are: Attack, Guard, and Break.
Attack hurts Break hurts Guard hurts Attack. If you use or counter with an effective command, you will gain an extra attack (3x in a row)/counterattack(if you chose a command that the enemy's command is weak against).
Skills are unique Servant abilities. They can only be used once per round. Skills can be unlocked through spending skill points from leveling or better relationship with your Servant.
Once per round, the Master can also support the Servant with Codecast, ranging from offensive to healing to support spells. Codecast is concurrent with a Servant turn so it does not take up an action slot of its own.
Skills and Codecast use SP. The Master only has an SP bar, as he/she cannot be attacked directly. Items and experience are gained at the end of every battle.
Most types of enemies have their own distinctive attack patterns, and memorizing these allows the player Servant to deal high amounts of damage, while receiving minimal damage in return. The more you fight an enemy, the more turns you can reveal making the battle easier.