Balista (standard; recharge 5, 6) * weapon
Ranged 30/60; + X vs. AC; 3d6+5 damage and target is knocked prone. The creature manning the balista must use a move action to roll for a recharge.
The bonus attack should be the basic ranged attack bonus of the person using the balista. I would consider whether this is a superior weapon, i.e. does the person get a proficiency bonus for using it? Something to discuss with the DM. The proficiency bonus should be +2 if you allow one.
The "recharge 5,6" is normally reserved for monster abilities, but why not use it for players in this instance? It represents that it takes a while to recharge the thing. The fact that the creature needs to spend a move action to be able to roll the d6 to see if it recharges means that it must remain there each round, though it still has its standard action so the combat is not lost for this PC.
The damage is probably comparable to many daily spells you have. You can modify the damage ouput if you feel it's inappropriate, and/or you can make it a "recharge 6" if you want it to be available less frequently (in which case it would mostly be an encounter power - who would want to remain in the balista for 5 rounds on average before it recharges?)
Sky