How exactly does Leona's stun ability work? When she enters a Battlefield with an enemy, does that enemy not do any damage? Is that it?

Based on your description, you are referring to Leona, Determined.

To answer your question: Yes, when Leona, Determined attacks, her ability triggers and stuns an enemy unit at that location. A stunned unit does not deal combat damage during that turn.

Here is the breakdown of how this works:

1. The Stun Effect

According to Rule 410.1.b, "A Stunned Unit does not contribute its might to damage in combat." This is the primary function of the stun: it prevents the unit from dealing combat damage for the remainder of the turn.

2. Timing and Sequence

When Leona, Determined attacks, her ability is a Triggered Ability that goes on the chain.

• Trigger: "When I attack, stun an enemy unit here."
• Resolution: The ability resolves, and the chosen enemy unit becomes "Stunned."
• Combat: If that unit is the defender, it will deal 0 combat damage to Leona because it is stunned.

3. Important Nuances

• "Here" Targeting: The ability requires the target to be at the same location as Leona when the ability resolves. If Leona is moved away (e.g., by an opponent's card effect) before her stun ability resolves, the ability will fail because "here" no longer refers to the battlefield where the enemy unit is located (see FAQ #46).
• Stunned Units: A unit can only be stunned if it is not already stunned (Rule 410.1.a.1). You cannot target an already stunned unit to trigger her ability.
• Combat Cleanup: Stunned units lose their stunned status at the beginning of the next Ending Step (Rule 410.1.a.2).
• Leona's Recall: If Leona attacks and the defender survives, Leona is automatically recalled to your base during Combat Cleanup (FAQ #25).

Summary: If Leona, Determined successfully stuns a defender, that defender will deal 0 combat damage during that combat. However, the stun does not kill the unit; it only prevents it from dealing combat damage for that turn.