There has been a previous topic on "Spells I'd like to see" under the Spells thread - that this should be under (although it has been dead for 1 month now)
however your Ballista as conjuration requiring a crew in theory could work - however it is a massive combo that is very weak on defence, as all your opponent needs to do is pop off one of the crew and splat Ballista useless - it would only really work for extremely large games with two or more arena boards, it focuses too much on relying on what you need to get out to get it to work - rather than what is happening in the arena
thats how i see it anyway