It is indeed a bug, the Armor shouldn't be counted as HP for that effect. This will be fixed in a future patch.
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Ankama Trackers
"Spectral Duelist" working as intended?
By SirPandaaa#7826 July 03, 2017, 15:26:38
AnkaTracker
The card Spectal Duelist (http://i.imgur.com/l4kNAyz.png)
If a minion has 2HP or less he will die. But if he has armor that combined with his HP is 3 or over, he will not die
(So 1 HP and 1 Armor will still die)
I thought there was a clear diffrence between Armor and HP.
For example if you have a card that says "Set minions HP to 1". This ingores armor and only sets the HP to 1. And heals cannot give back armor.
So is it intended for Spectral Duelist to consider armor when deciding what he can kill?
First Ankama intervention

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It is indeed a bug, the Armor shouldn't be counted as HP for that effect. This will be fixed in a future patch.
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just ran into this and was wondering the same - thanks for confirmation!