I mean, Zen-Oh's erasure can override a wish from the Dragon(as we saw when he perma-killed immortal Zamasu), but a wish from the Dragon can also override Zen-Oh's erasure(as we saw when it restored all of the universes Zen-Oh nullified.)
Don't know if that translates into them being able to kill each other, but it's certainly possible, as both seem relatively unbounded. So this could really just be a quick-draw scenario: whoever acts first, wins. /shrug
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"I am tired of Earth. These people.
I am tired of being caught in the tangle of their lives."