Hey Terrofacial, this Notetag should be able to implement what you are looking for:
<Card Summons Scope> If target.fieldCount() > 0 Creature If target.fieldCount() == 0 Battler </Card Summons Scope>
If you are running both Field Spells and Field Creatures, then you'll want to filter by Type:
<Card Summons Scope>
If target.fieldCount("ally", "Creature") > 0
Creature
If target.fieldCount("ally", "Creature") == 0
Battler
</Card Summons Scope>This will let you attack the Player/Battler when there are no more Cards of Type "Creature" on the opponent's Field. You'll need to give those cards the "Creature" Type by adding it to the Type List in the Card Types plugin and giving them the <Card Type: Creature> Notetag.
Hope that helps,
MythAtelier Team