It Ithilien Lookout's Response is triggered upon entering play (text below), can it be used to discard an enemy with text along the lines of "Immune to player card effects" (e.g. Smaug The Terrible)?

I guess the real question is, do cards in the encounter deck have text that can be applied?

"Response: After Ithilien Lookout enters play, look at the top card of the encounter deck. If it is an enemy, you may discard it."

Cheers!

This can impact immune enemies. Until the enemy enters play, their card text is considered inactive (unless explicitly stated otherwise). That includes the 'immune to player card effects text,' and cards in the encounter deck are not 'in play.'