Cards such as Snake Spirit that use poison as their skill gain virtually no benefit from levelling up the skill. If all levelling up a skill does is decrease the cooldown timer this in my opinion is a laughable waste on poison skilled cards. If you increase the skill on any of the other cards and decrease the cooldown you get an instant benefit, either healing more often, dealing dmg more often, increasing enemies attack cooldown more often etc... but in the poor case of the lonely snake spirit the dmg is not stackable and can not be applied so in terms of panzers, guild wars etc there is no benefit due the poison only being able to be applied once. Fair enough there is the point that the poison can be used in every leg of a multi level map but who bothers using skills on those these days? Auto roll please thanks!
I suggest that in this case you give the actual poison debuff dmg a buff to coincide with the benefit other dmg dealing cards get from levelling up the skill and dmg per spin. You cant even benefit from having a second one in your team because you cant stack the poison and there is nothing left to do with the duplicated cards...