I don't see Ballista as being overpowered because it is Zone Exclusive, so if the Warlord really wants to bring out more than one Ballista he is going to have to sacrifice playing many of his other Zone Exclusive conjurations in the center zones. Ballistas aren't even that difficult to destroy and they are only dangerous every other round. And even if Ballista is OP the Warlord needs to have some powerful cards considering he is the weakest mage at the moment.
The problem isn't that the Warlord would be using multiple copies, it's that OTHER spellbooks would. Especially defensive builds. The Warlord does need help. Which is why I wish this was a War Mage only card.
8 mana for a ranged attack of 5 with piercing of 3 every other round... It's really really powerful in this state, and if kept this way, almost every spellbook would benefit by having at least 2 in it.
Charmyna and others have already had this discussion here
http://forum.arcanewonders.com/index.php?topic=12949.0 and agreed that it's overpowered in it's current state.
One ballista is only a minor threat since you can move into its zone to avoid its damage. But stacking ballistae really hurts - especially because of the nature of the ready marker mechanic. Therefore, a solution could be to change the ready marker mechanic for all conjurations (e.g. only one conjuration may be activated before or after a friendly creatures action phase).
If ballista is made unique (and no change to ready marker mechanic), this might not solve the problem forever. In a discussion with murphy yesterday he pointed out that at some point we might have a couple of really good conjurations that are all unique (e.g. Hand of Bimshalla, Temple of Light if there are more cheap temples in the future, Wizard Tower, Ballista). Even though these conjurations are unique they can stack with other unique conjurations leading to the same problem we started with.
