Not just anyone can work a ballista. It takes a lot of practice. Good thing no one's around to interrupt you in Ainle, so practice all you want. Well, except the monsters...
There are two Sticky Bomb crates in the Duke's map. Touching one of these gives you three sticky bombs that you can use to blow up a part of the Duke. They recharge over time. Like other spear arm boss vampires, the Duke's spear arm will break-off. His spear range will be severely reduced, and he will attempt to use fireballs most of the time instead.
There are two Erg Pots in Area 1. First in the rafters up the stairs in the first room, second in the final room on a platform.