When the title says ACT, hit either A or D. You will have less than one second to act.
A stands for ATTACK, you will attack the enemy.
D stands for DEFEND, you will defend against an enemy's attack (if they attack).
If you attack an attacking enemy, you will take 80% of the damage.
If you defend against an attacking enemy, you will take 50% of the damage.
If you do nothing, you will take full damage from incoming attacks.
Click the button when you are ready.