There are at least two methods of using kill triggers.
The easiest is using an attached trigger, on page 1 of the trigger you'll have the option to make it a Kill trigger (I think it defaults to approach or destination for an attached trigger, so look for the drop down menu with one of those displayed.). On page 2 you'll have the option to set the amount of damage needed to fire the trigger.
You can also use doctrine language. To do this, create a standalone trigger and set it to automatic. Activate the doctrine language button, and select platform damage as the doctrine to use. There's no limit to now many doctrine tests the trigger can run, so you can test for as many kills/damages as you want with a single trigger.
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