Have the final Boss follow you as you notice the oversized crossbows. When using these (ballista), keep in mind that, depending on where and how close the Tainted God is, you may not get a huge amount of damage. You just have to worry about the long range attacks and sometimes the dark spawn attacking. However, if you have party members then you should be fine. They can battle the dark spawn and you can remain focused on the dragon or Tainted God.