Dimensional Pursuer Beloa:
First, we need to prove that the monsters we see here aren't all there is.
First, stay the Lillim Royal Knight, Lillim Slayer, Lillim Great Magus, Gigantic Healer, Nephilim Royal Guard, and the Nephilim and Gigantes to find out what they actually are.
The Wandering Dead of the Dimensional, Roaming Vengeance of the Dimension, Lost Dimensional Evil Thoughts, Lost Soul of the Dimension, Wandering Dimensional Spirit and other monsters that exist within the Dimensional Rift should appear.
Slaying the Dimensional monsters attacking through the rift, you'll obtain Spirit Fragments. Just collect around 100 of those for me. What do you think? Not too difficult, hmm?