Is drawing an enemy from the deck considered being attacked?

No. After all cards are drawn you start with the lowerst encounter number first and one by one you encounter them. If its a enemy it attacks you if you encounter it.

Why are you asking AmethystDragon?

it seems the wording for the assassin does tend to lend itself to being O.P. I couldn't find any reference in the rules to what all attacks someone except certain spaces and other characters. Just checking out all possibilites.

sorry, nm. it was answered in one of the erratas I see now. The assassin can assassinate any card or character using Strength(battle).

It's the Assassins only ability so not really overpowered at all!