That's the tricky part. Since you don't have the "Area fire" option, you pretty much have to target a unit on the far side of the bridge and hopefully when they hit it'll be in the middle somewhere. I've managed to do it a few times though, so it's not too bad. Just keep the trebuchet fairly close to the battle so it has plenty of range to acquire the target on the other side. If the cow lands too close to the fight your people will suffer too, and that would be decidedly unpleasant for the battle outcome.