Leaders and units - who casts what on whom?

[baboune]

Nagot Gick Fel said:
Depends on a lot of factors. Usually it's the one that'll benefit the most from the buff, or the one with the toughest units.

Hummm... There is no way to make sure a specific group is targeted. What goes into defining who is the target for a spell? (offensive or supportive spells).