Dungeons 2

Story related, can't be missed.

In the campaign map "Occupy Wall Street!", you will be introduced to the Unicorn as a new mini-boss enemy found in the Overworld. To get this trophy simply kill it. You will need an Orc at level 2, the Orc Chieftain, to take the unicorn out. To upgrade your Orc you need first a Fighting Pit found in Arena rooms. Simply drop an Orc into a Pit and the Orc will slowly train until he is upgraded into an Orc Chieftain. Each upgrade level of a monster takes up 1 additional population point so if an Orc needs 2 population points then an Orc Chieftain needs 3 so make sure you have enough population points.

The Unicorn casts a charm spell on your troops preventing them from attacking it. To overcome the charm spell, you either upgrade your monsters to level 2 to become Orc Chieftains which are immune to enchantments or get a free one from a cave in the Eastern part of the map. With the Orc Chieftain and the spell Battle Cry which does an area-of-effect attack to remove enchantments, you can then go kill the Unicorn.