Minions like the Skeletal Cleric and Frost Mage ... The Frost Mage can fire an ice bomb, which deals decent area of effect damage while also building up chill on enemies, which slows them down.

The Cast on Minion Death is a Witch trigger spell ... However, you can start by unlocking Flame Wall and Frost Bomb. These will be the primary spells that will cover most of the early game ...