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I managed to get this achievement with Silent.
The essential card for this achievement is Catalyst, which doubles an enemy's poison or triples it, if you upgrade the card.
Just use any cards that apply poison and then Catalyst. If you get more than one Catalyst card in your hand, or you have a potion that applies the effect of the next card you use twice (I don't remember how it's called), this is even easier to accomplish.
I also recommend to do it when fighting an Elite or a Boss, as normal enemies can die easily before you get their poison to 99.