How about a different implementation of a spell?

Protective Pebble Spell 1

Evocation Attack
Traditions arcane, primal
Cast 2 actions somatic, verbal
Range 10 feet Duration 10 minutes

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When you cast this spell you create three balls of force the size of a pebble that float around you head. For the duration of the spell or until all the pebbles are expended, when an enemy enters within 10 feet of you, as a reaction, make a ranged spell attack against the targets AC. On a success, you deal 1d6 force damage and the target must make a basic Fortitude saving throw. On a failure, the target is pushed back 10 feet. On a critical failure, the target is pushed back 10 feet and knocked prone. You determine the direction the target is pushed, and the pebble is expended regardless if the enemy is hit or missed.

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Heightened (+1) The number of pebbles increases by 1.