Not my guide 
!To obtain this trophy, it is necessary to summon the skeletal Dawn Warriors using the special command of the Dawn Sword + attack (D,R+D,D attack)
How to obtain the Sword of DawnThe sword can be obtained as soon as you are teleported
and is located behind a breakable wall in the area directly above the Throne Room, at the upper right end of the Inverted Castle's Reverse Keep area.
Once you obtain the sword, equip it in both hands (it is a two-hand sword) and execute the special command to summon one of the four warriors of the dawn (Archer of Dawn, Mage of Dawn, Warrior of Dawn and the Hooded Archer of Dawn) randomly. Continue casting the spell until you get the trophy.
This trophy is potentially
missable. If you exit the room without collecting the sword and then come back, it is gone for ever and you won't be able to get the trophy since there is no enemy that drops it.