Omega Shenron does not count. When you put anything from Dragon Ball Z into a battle, it breaks the scale. Breaking the scale makes it not count.

But, You kill it by making it die. If its not immune to radiation, then there you go. Dead time lord dragon. Anclagon was just big. But so is Godzilla. Godzilla is very very large. And Godzilla is immune to breath weapons, as the elements don't affect it, as well as the fact that it can withstand a very very very hard beating, from things much larger than it, that can fly, and just shrug the blows off and regenerate any damage dealt to it. Io and Tiamat also can't really be factored in because what they can do is based on the GM's playstyle. They can either be really strong or really weak, completely up to the DM. Dungeons and Dragons creatures can't effectively be put into a death battle.