I have read this part of the Mold Earth as allowing a trip effect:
- If you target an area of loose earth, you can instantaneously excavate it, move it along the ground, and deposit it up to 5 feet away. This movement doesn’t have enough force to cause damage.
The save is a Dexterity save against the spellcaster's Save DC or be knocked Prone. I had ruled that Huge and larger creatures cannot be tripped this way, and that Large creatures and creatures with more than two legs have Advantage on the save.
It seemed clever the first time it was used but now it creates problems with encounters — the Wizard simply Readies the spell until after the intended target's turn, and releases it with his high Spell DC (15- he has 20 Int at level 1). This allows the party members to attack with Advantage for the whole round.
Does tripping the enemy in this way violate the limitation of force exerted by the spell?
Congratulations to your player in coming up with an imaginative use for the spell. Congratulations to you for enabling it and working out simple, practical mechanics for it.
I personally would have made it a Dexterity (Acrobatics) ability check rather than a saving throw as it is an attempt to keep your balance rather then actually resist the spell and also because more creatures have proficiency in Acrobatics than Dexterity saving throws. I would also consider Strength (Athletics) to stand your ground and give the target the choice.
I would also rule that creatures with other than 2 legs get advantage due to their greater stability.
Is this OP?
Does it break the rule about causing damage? Clearly not.
However, what I think you are asking is: is this overpowered for a cantrip?
My take is probably not and here's why:
Readying action requires you to "decide what perceivable circumstance will trigger your reaction." That is perceivable to the character. "When it completes its turn" is perceivable to the player, not the character.
I'm not saying it can't be done, I'm just saying it can't be done with that trigger.
"When it attacks, moves or casts a spell" is a workable trigger but there is the possibility that it does none of those things and the wizard is left with nothing to hang their action on.