To earn the Lightning Rod trophy in Assassin's Creed Odyssey, you need to travel to Andros (where you upgrade your spear) and head towards the east side of the island to find a new mission waiting for you, provided that you have the latest Odyssey patch.
Here you will have to fight a second Cyclops enemy, much like the one that earns the below trophy:
Steropes appears to be packing quite a lot of damage but unlike the original Cyclops its a lot easier to manage your movements out in the open, and you don't have a collapsing ceiling to deal with. Otherwise his attacks are pretty similar. Just be patient and bring fire arrows along with fire damage and you'll find this quite easy at level 50 (Steropes appears to be capped at 46 while the player is at 50).