Destroy the 1 face-up monster your opponent controls that has the lowest ATK (your choice, if tied).
Rulings
Individual Card FAQs
You can activate "Fissure" even if your opponent only has 1 monster on the field.
If there are two or more monsters with equal ATK on your opponent's side of the field, you (the player who activated "Fissure") select which monster will be destroyed.