Inferno Fork

The fireball is not really the weapon. What matters is what it leaves behind: a wide burning cloud at the point of impact that sits there and keeps hitting everything inside it. Aim at a wall or the floor near a group rather than at any one enemy, since hitting terrain triggers the same cloud and gives you control over exactly where it forms.

Damage scales with how long enemies stay in the fire, so knockback was deliberately toned down to stop targets being punted out of their own burn. Corridors, arena floors and doorways are where this goes from good to unfair.

Events. Anything that sends a continuous line of enemies at you, Pumpkin Moon waves or a Dungeon full of respawning skeletons, lets a single cast do the work of many. Against one boss that keeps moving it is much weaker, because the cloud stays where it landed and the boss does not.

It is one of the few fire weapons that still functions underwater, though the projectiles crawl there. Farming it means grinding Diabolists in the Hardmode Dungeon, which is its own unpleasant job given how they teleport and how hard they hit.