How do you kill a summoned phoenix?

Everyone here was discussing how to defeat a phoenix using summoning or chaos magic. However, it's unlikely that a Knight needs these skills. The most optimal option is...to upgrade the Ballista! And it will tear the phoenix to shreds, even if it has more than 1000 HP.
Level up the "Expert in War Machines" skill, then the Ballista, then the "Triple Shot" skill for the Ballista.
You can also level up the attack skill, take "Archery," and then "Greek Fire."
In the end, you get a VERY powerful Ballista.
And for an even more devastating effect, you can use the hero Vittorio.