First of all, look carefully at the the bottom of your screen once you enter the minigame.
You can see 2 types of symbols on the bottom of your screen. Small purple lightning symbols are your Health Bar whereas the big symbol is indicating your Energy to either attack / block.
Attack and / or Block both consumes Energy, but it will refresh in a few seconds after being used or you can drag your Sword towards the Energy Refill symbols that randomly appears on your screen during the minigame to refresh your Energy instantly. Even though your Energy will refresh in a few seconds, be mindful when you want to use it to Attack as you can't succesfully block an enemy without having Energy.
All of the enemies have 3 Attack Patterns, which are Small Circle ( Quick Attack ), Big Circle ( Slow Attack ) & Red Bar ( Wide but Slow Attack ). To block the Circles, drag your Sword on top of those Circles and click the right-button of your mouse nearly or exactly when the timing is right ( Similar to rhythm game ). As for the Red Bar, drag your Sword away from it and use the Energy to Attack the enemy from the safe area instead by using the left-click of your mouse.
Again, be extra careful on how you use the Energy to either Attack or Block. For example, after you Block once you want to Attack next. However, after you use the Energy to Attack, you see a Small Circle appear on your screen, indicating the enemy use Quick Attack on you. If you're lucky, the Energy will be refreshed right before you received damage and you successfully blocked it. But most of the time, you'll receive damage first then the Energy get refreshed after.
The safest moments to Attack is when you see the enemy use either Big Circle or Red Bar Attack, since both of those are relatively slow so you can successfully Block ( Big Circle ) after or just Dodge it right away ( Red Bar ).