Trying to change camera focal point based on objects near frustum

I'm wondering how to approach this issue (3d side scroller) c#

Right now my camera is focused on targets either in front of or behind the player, depending on which direction the player is running. If there is an enemy, I will focus on a point in between the player and the enemy.

What I'm having trouble with is this: I'd like to change the camera's point of focus slightly before the enemy is in camera view. Sort of like, you're running, you're running, now suddenly the camera adjusts to show there is an enemy on the path before the enemy is in its own attackPlayer range

Checking the distances between the player and all the enemies in a large environment seems like its over-kill and way too processor intensive.

What I think would work well is - if there's an enemy within 5 units (or w/e) of the camera's frustum, so right outside the view, switch the camera's focus to the new position (which would be half way between the player and the enemy). Does anyone have any suggestions on how this could be accomplished?

 

bump - any ideas?