You can click card to select or click again to deselect (selected cards will be lifted or pushed up) Whenever you done selecting cards you want to use for your attack, then simply click monster to execute attack. You can still attack monster without selecting any card - in this case it you are attacking monster with basic attack. Basic attack does not consume weapon's durability so it is best to do so whenever your weapon inventory is low but without using cards or executing combo, you can't earn any training points so that is the trade-offs you have to deal with. Whenever you selected multiple cards, you are trying to combine attack (or executing combo attack). Make sure your choice of selected cards are valid one or else you can't attack monster.

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System - Battle

KEYPAD NAVIGATION

Keypad Description
Up, Down, Left, Right Move character
A Bring up Ability screen
S Bring up Status screen