Well, from my dabbling in chance on hits, sometimes it can get confusing trying to remember who is who in a situation. The only way I've consistently gotten triggered chance on hits to work is to make a trigger based off of "a unit is attacked" + "attacking unit is <insert unit>" = "If : Level of the unit's chance on hit spell > 0 and random number between 1 and 100 =< whatever percent chance, then do the chain lightning or aoe or whatever the unit is going to do. My example goes off the page a long ways from arithmetic because of a complex number involving %current mana, level of the ability, etc. so I'll just post a simpler, easier-on-the-eyes version.
Event -
A unit is attacked
Conditions -
(attacking unit) equal to (unit-type(insert unit here))
Actions -
If/Then/Else (multiple conditions)
-Conditions - Level of (Chance on Hit Spell) of (attacking unit) is greater than 0
Random number from 1 to 100 less than or equal to 10 (or whatever the chance on hit is)
-Then - cause (attacking unit) to damage (attacked unit) for (however much you want in however big of an area)
-Else - leave it blank
Where it says deal damage, you can always tell it to create (dummyunitblah) at 0,0 from (attacking unit) facing (attacked unit) and then make the dummy cast whatever spell the chance on hit was gonna do.
If this is still confusing, I'll gladly try to recreate what you need over the weekend for you and add some trigger comments to tell you what each little part does. I hope that helps you for many maps to come.