For those who don’t know, when you purchase Potion of Armor and get 25% Skull Damage Reduction, this stacks with any existing skull damage reduction on the troop or your hero. It is not additive however. So if a troop already has 25%, it won’t have 50%. Instead, the incoming skull damage is reduced twice. Here’s a table showing the amount of damage blocked at various skull damage reduction levels based on 100 incoming damage:

Base Skull Damage Reduction | Incoming Damage | Potion of Armor Reduction | Total Skull Damage Reduction | Flat % Increase |
---|---|---|---|---|

0% | 100 | 75 | 25% | 25% |

20% | 80 | 60 | 40% | 20% |

25% | 75 | 56.25 | 44% | 19% |

33% | 67 | 50.25 | 50% | 17% |

40% | 60 | 45 | 55% | 15% |

50% | 50 | 37.5 | 63% | 13% |

65% | 35 | 26.25 | 74% | 9% |

80% | 20 | 15 | 85% | 5% |

Potion of Armor is clearly most effective for troops/heroes that have no skull damage reduction, because it reduces the damage by 25% when it wasn’t being reduced at all. So the total reduction increases a full 25%. However, as your base skull damage reduction increases, the “effectiveness” of the 25% decreases.

For instance, Stonehammer has 80% skull damage reduction. So the amount of damage that gets reduced from Potion of Armor is already really low. That’s easy to see: if you get attacked for 100 and it’s reduced by 80%, you have 20 incoming damage. If it gets reduced another 25%, it only gets reduced by 5, down to 15. But if your original skull damage reduction is only 40% (Holy Armor), then your damage taken is 60, and 25% reduction on that is 15, so you only take 45.

This means for instance that if 50% skull damage reduction normally is good enough, then you could get by with a troop that has only 33%, because with the 25% Potion, that’s a total of 50% damage reduction.

Of course, at higher delve levels, you’ll want all the damage reduction you can get…