I liked this map, it was sweet and to the point.
As for challenge level, why not make it grow stronger with every beam, until the set level of creeper, and it dies because of overloading? This would allow the player to tackle it, but if they don't start to up the defense, the beams in range could be at risk, allowing for more of a fight and making this foe almost the same from encounter to encounter. Since the foe would not be target-able from nulifiers, the risk of having one low-medium emitter would not be there.
I do have a quick little question, when I was looking into making custom entities I had trouble displaying more than one line of data. I tried to just copy the example and then fill in the info relevant to the entity (in this case a "Terrain Rail Walker" (from the sample page).
I could only display one height value and on;y on one line...
$MIN_TERRAIN_HEIGHT:1
$MAX_TERRAIN_HEIGHT:3
$SPEED:1
"Max_Hieght:" <-MAX_TERRAIN_HEIGHT concat
"Min_Hieght:" <-MIN_TERRAIN_HEIGHT concat SetPopuptext
once
only the min shows up
Any help in this matter would be helpful to me.