When using Rotom's "Plasma Arrow" attack, I get to choose one of the opponent's Pokemon and do 20 damage for each Energy attached to it; but do I do the damage to the Defending Pokemon, or to the Pokemon I chose?
To the Pokemon that was chosen. If the damage were to be done to the Defending Pokemon, there would be a "20x" value in the damage field of the attack.
Source: Undaunted FAQ (2010-09-09), PUI Rules Team (2010-09-09)

Relevant cards

Note: All applicable cards may not be listed below.
HGSS3_20