The Wizard at Fifth Level
The Wizard scales in steps, not a smooth line. The first meaningful step is character level 5. That is when 3rd-level spell slots open, and with them Fireball: 8d6 fire damage in a 20-foot-radius sphere, Dexterity save for half. Average roll is 28 damage to everything in the zone. Against a pack of low-HP enemies, that is often the whole encounter.
The common failure is habit. A Wizard spends levels 1 through 4 leaning on Fire Bolt and the occasional Magic Missile because slots are scarce. At level 5 the math changes and the habit does not. Firing a cantrip for a handful of damage while a 3rd-level slot sits unused is a waste of the class’s defining feature. Slots recover on a long rest. Dead party members do not.
Positioning matters as much as the slot. A 20-foot radius catches allies who wandered in, and Fireball does not discriminate. The spike is real, but it rewards a Wizard who watches the board before committing the sphere.
At the table: At level 5, when three or more enemies cluster within 20 feet and no allies are among them, spend the 3rd-level slot on Fireball rather than a cantrip.