What levels of what troops can you kill with one poison spell and does upgrading the spell allow you to kill more types of troops - in terms of health?
A level 1 Poison Spell can easily kill all Tier 1 troops as well as any level Wizards, Witches, Skeletons, and Minions. Upgrading this spell, however, is still useful, as higher level Poison Spells kill them faster and leaves more time to finish the attack......A Poison Spell deals scaling damage, which can take a significant amount of time, depending on the type of Troops inside the Clan Castle, so make sure that you have enough time left to finish your attack after the Clan Castle troops are dealt with.....As of September 18, 2015, Poison Spells now do scaling damage in a similar manner to a single target Inferno Tower, just to a lesser extent. This scaling effect dramatically affects high hit point units, but does not deal the same scaling damage against Heroes, instead dealing little appreciable damage.
Edit: I was able to test this with a max balloon. Once the balloon was pulled out of the opponent's clan castle [and stationary], 1 level 2 poison spell was able to take out a max balloon and do half damage to max dragon (that was also in the poison's radius).