How do I make a troop event happen once?

I know how to make the enemy say dialogue when he's below 50% health, but he starts repeating the dialogue every single turn below 50% health rather than doing it once. I've got no idea what to do.

Set the scope of the troop event to 'Battle'.

EDIT: I just realized this is for RPG Maker 2K/2K3, which I haven't worked with before. I assume there is a 'Battle' scope, but if there isn't, then here's what you do:

1. Have an event run on Turn 0 (or at the earliest possible time) and set a Switch to OFF
2. Put the whole event body (what is currently inside your troop event) inside a Conditional Branch that requires that Switch is OFF
3. Turn that Switch ON at the end of that event body

This will ensure that even if the event technically runs every turn, it will only actually appear to the player to run once per battle.