how do i make a "Poofer" ?

[FlawlessHappiness]

String would be monster name.
Bool is always true or false. Things like Int and Float are numbers.

SetEnemyIsHallucination("monstername", true);

As for distance where it vanishes, I have no idea how that's calculated with this script. My guess is that as soon as the monster is triggered to be a hallucination it would vanish wherever it is. Without testing though I don't know.

NECROOOOOO!

To make it easier, you can tick this in the editor. Just go to the monster properties and tick "Hallucination". This is also where you can specify the actual distance from you to the monster where it would disappear.