I'm looking for some advice on what type of ballista (A launcher type thing, just in case ya didn't watch enough Junkyard Wars growing up.

) design would be best for launching a water balloon a minimum of 20 yards. Accuracy is important, as we have to hit him for full credit. Also, compressed air, chemicals, and motors are disallowed. The water balloon will be roughly softball size (the teacher hasn't specified exactly yet).
I only have 10ish days to build and test it, so simple is good.
The classic hand slingshot hasn't been specifically disallowed yet, but assume that you can't use that. (But we can probably get away with a mounted version of that with some special mechanical release.)
What sort of device would you recommend and is there anything we should look out for?