Worlds of Magic: Planar Conquest (UP)

The description on this trophy is horribly vague; you cannot believe how many scenarios I tried before I discovered one that worked. Even with the solution mentioned below, it can be a time consuming pain to get the right conditions set up.

The secret to this trophy is that the description is not talking about killing an enemy hero in a battle. Some spells are only usable while in Battle mode and others are only usable while in Strategic mode (your normal state). This trophy wants you to kill an enemy hero while using Strategic spells on the World Map.

There are some high level Strategic spells that will cause damage to units. I will not list all of them as I can only confirm the one that I used to unlock this trophy: Air/Biomancy Tier VIII - Corrupt Atmosphere.

Before I cast the spell, I made sure that there was an enemy hero on the map. The opponent had also changed our Diplomacy to War. Casting this specific spell causes 6d8 damage to every single unit on a specific plane during every following turn. This includes your own units so realize you are going to wipe out your own armies doing this.

The trophy unlocked when 5 turns had passed after casting this spell, even though I could not see the enemy hero through the fog of war. I know they had 65 hit points.

The random element to this trophy is waiting for any opponent Sorcerer Lord to acquire a hero while you take your time researching spells in Air. If you focus specifically on only researching Air spells then you are certain to acquire Corrupt Atmosphere eventually. Note that if one of your opponents is Markas they always start with a hero.