Strategic Spells impair a Foe or enhance your Teammates in a Wizarding Challenge before you start combat. To use a Strategic Spell, you must first upgrade your Profession Lessons by using Scrolls, Spell Books, Restricted Section Books, or any combination of them.For example, to acquire the Strategic Spell, The Weakening Hex (shown below), the upgrade costs 4 Scrolls.
After you acquire a Strategic Spell, you can upgrade that Strategic Spell to make its effects greater and increases your proficiency in Wizarding Challenges. Additional upgrades will share the same symbol of the Strategic Spell it enhances.Using Strategic Spells in Wizarding ChallengesBefore you engage a Foe in combat, you can use Focus resources to activate Strategic Spells that will impair a Foe or enhance a teammate. Your Strategic Spells appear in the spell tray at the bottom of the Arena screen. You can tap a Strategic Spell to see the description of its effects and the amount of Focus it will consume.
Acquire more Strategic Spells to fill in the empty slots that appear with a lock.To use a Strategic Spell, drag it from the spell tray at the bottom of the screen to another player or Foe. Drag a different Strategic Spell from the tray to the same player or Foe to swap the spell. You will see a transfer icon as you hover over the Foe with the new Strategic Spell before releasing the spell. You do not receive Focus back for switching spells.Other Strategic Spells that can be used repeatedly have a cool-down period after you use them. You’ll have to wait before they are active to use them again.
The Focus meter at the bottom indicates the amount of Focus you have. Strategic Spells consume Focus to cast before combat. 
You can replenish your Focus meter by defeating Foes. Foes that appear with a Focus icon will award you and your teammates with more Focus when they are defeated.
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Harry Potter: Wizards Unite Game GUIDE
哈利波特：巫師聯盟