[DGDobrev]

Basic formula:

Damage = Burn/Poison Factor * random number (0.05-0.1) * Current total troop health * Medal Bonus

where:
1) Burn/Poison factor is equal to - Leadership of the troop who inflicted the post-effect/Leadership of the Target
Restrictions: min 0.1, max 1.0. The factor can only range between these 2 values.
Special: Burn/Poison factor for spells is always 1.0. It is also 1.0 for the summoned Phoenix's ability Fire storm.
2) Current total troop health - the current total health of the troop that has received the post effect.
3) Medal bonus - the bonus from the Battle Alchemy medal. According to the level of the medal, the bonus is 10/20/30%, so the multiplier will be 1.1/1.2/1.3.
Restrictions: if a friendly unit is poisoned by friendly unit's attack that has a post-effect, the multiplier is 1, regardless of the level of the medal.

There you go. These are the game mechanics. I know that KBTL had it different, but this is how it's done in KBAP. In this case the current total troop health was your biggest problem. If it were 1 snake, it would have burned for 1 point.