I've attached a map which implements chain cripple. The basic method employed is to use a chain lightning that does very little damage, like 0.04. So little damage that almost nothing else could be causing the damage, making "Unit takes damage" a fairly reliable event to use. So in effect it can detect the targets hit by chain lightning or shockwave.
More details are included in a trigger comment. This method is a pretty good way to make a few non-damage dealing AoE abilities in a map. | 
