Targeted Unit?

When I cast a spell, say Firebolt at another unit, and I want some random thunderclap effect to play on the targeted unit, which condition would I use?
Target Unit of Ability Being Cast or Targeted Unit?
I tested both, but neither worked...

Target Unit of Ability Being Cast should suffice. I'd say that the problem might be on another part of the implementation of your spell. Check other parts of your trigger and spells. It could be as simple as not giving the command to cast thunderclap at all.