Under normal circumstances, the monarch-level opponents that spiritualists can defeat are usually weak monarchs, very rarely standard monarchs. As for strong monarchs and monarch peak... basically, no one can defeat them.
Weak, standard, strong, and peak are the classifications spiritualists use for bosses of the same level.
"Let me tell you, the monarch peak boss he defeated was quite something. He was once the Child of the World, and at his peak, he even possessed dominator peak strength, the strongest Child of the World in history."
"In the darkest times of that world, the remnants of ancient gods birthed three terrifying ancient calamities, all dominator level. Calidos stepped up and killed all the dominator-level ancient calamities, challenging the source of the calamity alone."
