I'm trying to do this but I'm not successful. Can anyone help me?
"The HUD should only be displayed when the combatant’s HP aren’t full? Use a Status Requirement in the HUD’s Display Conditions to check for the HP being less than 100 %.
To only display the HUD for a short amount of time when the combatant received damage, you can use Status Effects. E.g. create a Damaged status effect that ends after a few seconds and is added by all abilities that deal damage. Check for that status effect in the Status Requirements of the HUD."