Here's some background info that may help. The plugin iteratively checks each enemy during a screen update. You can access each individual enemy object with this code, where i is the index that the plugin uses to iterate:
$gameTroop.members()[i]
As such there are tons of different ways to show or hide the HP bars for each specific enemy in each scenario. For example, this is a condition I use (in addition to some others):
$gameTroop.members()[i].isSelected()
This checks if an enemy is being selected by an attack, skill, spell, etc. If you wanted to only show HP bars for living enemies, try this:
$gameTroop.members()[i].isAlive()
This method does exactly what you think it does, and only evaluates true if the enemy is still alive.