Enter the Dungeon: "Well of Eternity" an keep on going till you are fighting Manoroth an Varo'then.
When you defeated Varo'then, just keep hitting the spawning deamons while Tyrande ist killing them.
If Manoroth allready spawned the "Big Green" Portal, you will gain more Grisly Trophies.
Illidan can stand against Manoroth quiet a long time, and your group can farm Grisly Trophies until either Illidan or Manoroth is defeated.
Test an Enjoy.