In "The Survivors," the Douwd tries to scare away the Enterprise using a re-creation of the imposing Husnock ship. Why not use something more intimidating, such as a Borg Cube or the Crystalline Entity? Perhaps two or three Romulan Warbirds?
Uxbridge had just seen his wife of many years brutally murdered, seen the colony he lived in destroyed and then borne the unimaginable guilt of killing 50 billion people in a fit of rage. He was shell-shocked and insane, trying desperately to hide from reality. Resurrecting the Husnock ship is the best fit for both his battered state of mind and the evidence all around him of the destroyed colony. Maybe in other circumstances he would have been more clever, but mental acuity isn't what you would expect from someone who had just suffered such crushing psychic injuries.
Perhaps the presence of the Borg or the Romulans in orbit around a Federation colony world would have invited further intervention from the Federation, something the Douwd wanted to avoid.
It's also possible the Douwd simple hadn't encountered all of the intelligent species, such as the Crystalline Entity, previously seen on Star Trek and therefore couldn't make a convincing mental projection.
The Douwd was, at heart, a pacifist. He didn't think in terms of war or destruction or intimidation. If he did, then the whole story, even before the Enterprise showed up, would probably be different. He doesn't think in terms of battleships or weaponry.
Also, as one other answer pointed out, he had just been through a terrible emotional trauma. He wasn't thinking logically. He took the tools that were in front of him. The Husnock battleship image was still fresh in his mind and was probably the easiest threat to recreate due to recency and the emotions attached to it.
Even at this point, he wasn't thinking in terms of harming anyone. He just wanted them to go away. He started with a simple, "I don't know why they spared us," and from there his attempts escalated. He was trying to use minimal possible force as well as attempting to discourage continued interaction.
Logically, once he and his wife were destroyed, the Enterprise would have no reason to remain. That's what he expected and that's how he played it.