Just checking on this, 'cuz I can't remember exactly:
How does this work for a stack of units? If there is more than one unit with the "Support Fire" ability defending, do they all do their support fire, or only one? If another unit attacks that plot, can that same unit do another support fire, or does another support fire unit have to take up the task. Likewise, if a plot already had one unit use it's support fire option, can any other unit use the support fire option that turn? Finally, what represents a turn? Is it a player's turn, or the unit's player's turn. In other words, if player1 gets attacked by player 2 and unit X uses support fire, then on player3's turn the same plot is attacked, can unit X still use support fire?
Any other scenarios I should know about? Really, the battle mechanisms are the part I still haven't dug deep into, so this would be an interesting challenge.