Snowball mentioned everything, but didn't mention the Sylvan heroes and their Deadly Shot ability. Also, just like the Striking Sword of the Inferno lords, it guarantees the death of one unit.
The only thing is, I'm not sure if the Phoenix needs to be on the hero's list of sworn enemies for this to work. IMHO, killing a week's worth of Phoenixes is not an easy task, as there aren't that many of them.
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