Can you react with Rengar, Pouncing to attack an enemy battlefield solo?

Yes, you can play Rengar, Pouncing as a reaction to a battlefield you are attacking, even if he will be the only friendly unit there.

According to RiftJudge FAQ #8779, Rengar's specific card text ("I can be played to a battlefield you're attacking") provides an exception to standard unit placement rules. Because he also possesses the [Reaction] keyword, he can be played at reaction speed during a combat showdown (a closed state).

Important Considerations:

• Solo Entry: You are permitted to play him to a battlefield you are attacking even if you currently have no other units there.
• Attacker Status: When Rengar enters the board during a combat showdown, he does not immediately gain the "Attacker" designation. According to RiftJudge FAQ #9610, he only gains the "Attacker" designation and activates his Assault 2 keyword during the combat cleanup triggered by his entry.
• Timing: As a unit with [Reaction], he can be played while a chain is already on the board (a closed state).

Sequence:

1. Combat Showdown: A combat showdown is ongoing at a battlefield you are attacking (the state is closed).
2. Play Rengar: You play Rengar, Pouncing as a reaction. Because he has the [Reaction] keyword, he is played at reaction speed.
3. Entry: Rengar enters the battlefield.
4. Cleanup/Trigger: A cleanup occurs because a permanent entered the board during a showdown. During this process, Rengar gains the "Attacker" designation.
5. Assault: Once he has the "Attacker" designation, his Assault 2 keyword activates, increasing his Might.