The trophy is awarded upon defeating the first boss of the Boletarian Palace archstone: the Phalanx.
The Phalanx is a giant harmless blob that doesn't have any way of actually hurting you. Instead, you have to fend off the 30-40 Hoplites that cover its body. The Hoplites are the smaller blobs with a shield on the front of their body and a spear on the right side. They can, and will, throw these spears at you if are out of melee range. Thankfully, there are many pillars lining the sides of the room that you can hide behind to avoid being hit. Alternatively, you can roll out of the way or block them with your shield.
The Hoplites are very weak to fire, and since they are all crowded together, it's easy to kill 4-5 at once with a single firebomb. If you run out of firebombs or don't have any to use, you can hide behind one of the many pillars, apply turpentine, then run around the boss and attack him directly. If you have no firebombs or turpentine (or would prefer not to use them) then lure the Phalanx in close while hiding behind one of the pillars and when the Hoplites detach, run around the Phalanx and attack the opening in the back.
If you started the game as a magic class (Magician or Royalty) then you have access to either Flame Toss or Soul Arrow, both of which can be spammed to easily take out the Hoplites from afar.
Defeating the Phalanx earns you the Lead Demon's Soul and the Phalanx's Trophy.