QIf the only enemy followers on the field are a Heavenly Aegis and a Goblin, and I play this spell for 5 more play points, can I banish the Heavenly Aegis?

AYes. You can select the Goblin and play this spell, which would then banish each enemy follower on the field, including the Heavenly Aegis.

QIf the only enemy follower on the field is a Heavenly Aegis, and I play this spell for 5 more play points, can I banish the Heavenly Aegis?

ANo. If you can't select an enemy follower on the field, you can't play this spell.

QIf I play this card for 5 more play points, do I still need to select an enemy follower on the field?

AYes, you do.