How do I cast a spell on a monster?

To cast a spell on a monster, left-click on the monster so it is targeted. You will know when the monster is targeted since it will display the monster's portrait & information next to your own. Once the monster is targeted, you can cast a spell either using one of your hotkeys (if set up), or directly from your Spellbook (P) and clicking on the spell's icon. Please note that you cannot cast any beneficial spells on monsters (heals, buffs, etc.).