Catalyst

Stack 99 or more Poison on a single enemy. Silent: build Poison (Noxious Fumes, Deadly Poison, Bane) then play Catalyst - it doubles the target's current Poison, or triples it upgraded. Get a target to around 40-50 Poison and an upgraded Catalyst punts you past 99 in one card. A high-HP enemy or boss gives you the turns to stack it. You'll very likely get this naturally on the way to a Poison win / Emerald.