So I mean add a condition, which appear on the left (press C), to the on collision event. The condition should be critter opacity=100. This means when the bullet collides that the critter object opacity must be 100 for it to do the actions i.e. it is not fading. So you can only hit it one time with that condition.
Sorry for misunderstanding, I'm still new to this and often get the terms mixed up. I've tried adding another condition but can't find the "set opacity" option. The closest I can find is "compare opacity".