Lightning does not do damage. It simply kills a part of the enemy stack just like Reaper's soul draining ability in KBTL. As such, it is affected only by spells or abilities that may improve the damage a target takes (of any kind). Still, like Reaper's ability, it shows the damage amount based on the total health of the units in the troop it killed.
Well, ok, I'll tell you what I did. The Lightning would have killed about 44-45% of the stack anyway. That's a pretty high percentage. But since I was aware that this stack is extremely dangerous, I had to make it kill more of it. So I added mark of blood to make it kill 90% of it. The damage was about 85k, which was great, but it wasn't tremendous. So I started looking for ways to increase it through other spells. Peacefulness seemed to be the best one available, so I tried it and that was the result. Doom doesn't work, as it only affects weapon-based attacks. Helplessness also doesn't help, since the lightning isn't affected by it.
So I confess
That was a simple trick to "improve" the "damage" by improving the enemy stack's HP. If I had lvl 3 peacefulness, the damage probably would have gone even higher. That was simply fooling around with the game mechanics.
I think peacefulness is the only buff spell that can be cast on an enemy squad. I may be wrong though.
And it does. I loaded back the save game, got lvl 3 order (which I wasn't going for at all, this is why I had only lvl 2) With level 3 peacefulness, I managed to nudge it like 13k higher. Here's the screen. Maybe if I fool around with the game a little I may even make it go over 150k.
The general rule of thumb is: total health of the troop is what matters. If the total health is 200k for example, and you put peacefulness lvl 3, the health will become 300k (+50%). So if you put seal of blood, ball of lightning will kill twice as much units. If you luck out with a percentage of 48% of the units, this means 48%x2=96%. 300 000x96% = 288000 points of health destroyed = 288000 damage. This is what the game will show.
EDIT:
I can go in-depth here.
There are 1552 Gorguls, 70 HP each. That is 1552x70=108640 total troop health.
I put on peacefulness, which improves the stack's HP by 50% and reduces damage by 30%. The latter is irrelevant.
108640+50% health (1.5)=162960 total troop health.
I put on mark of blood. That forces any damage the troop takes to double.
Ball of lightning kills up to 48% of a troop MAX. With Mark of Blood, that goes up to 96% MAX.
162960x96%=156442
With Peacefulness lvl 3, 1 gorgul's health becomes 105.
156442/105= 1489.92 => I can kill 1489 gorguls at most from this troop.
156442 - that is the highest damage this troop can take from Ball of Lightning in the best case scenario.