If I have a Liepard with 3 Darkness Energies on it and I use "Assist", can I use Charizard's "Scorching Fire" attack despite not having a Fire Energy to discard?

Yes, you can. The damage amount is done regardless of whether Liepard has any Fire Energy attached to it or not, but if you do have a Fire Energy attached to Liepard you must discard it.

Relevant cards