I have the dragon create it as close as possible to the stack I want it to hit. However, if nearby has a stack of similar health, which in AI's opinion can pose more threat, the lightning will attack it and not the stack you want. but, provided you have mana, you can spawn another ball of lightning and the second one in this case usually hits the stack you want.
It is a bit dodgy, but once you get to know the ability, it is very easy to control it. You should be aware of some restrictions:
The lightning has speed of 5 (I think)
After the lightning has attacked or moved, a charge is lost
A lightning can do only 2 actions (in the beginning) or 3 (at higher level)
Damage is really low against small stacks
If you have 2 or more balls of lightning, 2 cannot hit the same target in one turn
Once you get to know the ability, you're going to love it. I, personally, use it all the time, sometimes even twice (or even three times as a mage) in a round, especially if the enemy army comprises of severa stacks with 200+ units.
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