I know ballista is a promo. Still, it might be good if we, the players, have a deeper look into it in theory as well as in real games before its released.
Compared to creatures in that cost range, Ballista does twice the damage but can attack every other round. So on time average it seems like it does the same damage. But, you need to consider that the Ballista starts with a load token. Therefore, the round after its summoned it does twice the damage of a creature!
Here are a couple of games in which I used a ballista focused opening:
The first one is me playing a Fire Wizard against a Warlord. I summoned 4 Ballistae until round 3 whereas the Warlord used Brogan. Even though I had quite some bad rolls and the Warlord had decent rolls against the Ballistae, the damage from the ballistae combined with the wizard towers utility was too much.
http://www.twitch.tv/charmyna/c/2795140This one was vs a BM:
http://www.twitch.tv/charmyna/c/2795732And one against an air wizard with Grizzly+Vamp:
http://www.twitch.tv/charmyna/c/2796603Here is a really tough game against Fentum (Air wizard). He destroyed two ballistae quickly with wizard tower + fireball. After that he pushed me through wall of thorns twice (I couldnt do anything because he had ini next round). In the end I survived with 1 life left!
http://www.twitch.tv/charmyna/c/2796757This video somehow shows that fire+2 is an important weakness of the ballista. Still, mana and action wise its inefficient to kill ballista with fireball since the chance to not instantly kill it isnt that small. Even if you oneshot the ballista, it might had the chance to shoot once depending on Ini.
A solution might be to start with zero load tokens or to make ballista unique. 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.