If the monster doesn't have enough range to reach the attacker then "the Monster cannot win the battle; any result that would lead to a win for the Monster is instead considered a draw" (12). Other than that, the roll plays out normally. You still roll against its defense, only exception being that if your result is lower than its defense you don't get a wound.

For Area Effects, it states that you only take the highest defense of that monster that is directly targeted. A two target fireball wouldn't take the 4th monster's defense. Likewise a non-undead wouldn't count against for Turn Undead. Here's the Giant's ruling on TU against Xykon.
Quote Originally Posted by The Giant View Post
"Turn Undead specifically says right on the card that you can't use it against Xykon. Treat Xykon as simply not being there for the purpose of using that shtick; the card just ignores him. Note that you CAN use it at Range 0 even if Xykon is the top monster, it just doesn't affect him. Technically, the top monster IS undead, even if it is a monster with a specific immunity to the shtick, making the conditions needed to use the card true. Thus, you can use the shtick.

Turning Xykon's undead out from under him is a great strategy, by the way. You bypass Xykon's ultrahigh Defense because he cannot be affected by the card!"