Rom, the Vacuous Spider - Defeat Great One: Rom, the Vacuous Spider.
This boss and trophy are fairly easy to unlock, Approach Rom and begin to attack him until he teleports away. He will summon smaller spiders. Begin to take out the small spiders so it is easier to attack Rom. Do this multiple times until Rom is dead.
The main attacks to watch out for are the homing ice hail and when he rolls over and his AOE ice spikes.
In no time you will have defeated the boss your trophy should pop! Enjoy!