Spawning and naming.
A step spawns a mob of a kind at a place, with a name and a tag, and can keep hold of it for the steps that follow. Tags are words the challenge puts on mobs so it can find them again.
Acting on them.
Mobs and entities are pushed, pulled, launched, mounted, set on fire, frozen, healed, hurt, killed, made invulnerable, given attributes such as speed, scale, and gravity, and made to glow. A step that acts on players acts on mobs too when it is told to.
Finding them.
The mobs near a place, the entities near, the dropped items near, the mobs tagged a word, and a mob's kind, health, and tags are values. Their events: spawned, hurt, died, and killed by a player.