You’re going to want to head waaaay back to Orlen, and head straight south to the very bottom of the map. Catch some Frostjacks, and DON’T evolve them or level them at all. They’re needed for fusion. Also, go back to Windon and search for some Cherubs.
Voltyke works too, as its final evolution is fantastic for boss battles.
Frostjack can fuse with Cherubion to create Luxknight, a very powerful 9.5* angelic monster. Its moves are Bladedance, Doubleslash, Glare, and Luminate. It also has great stats for a holy arkadion.
Frostjack can also fuse with Skullwraith to create Luxknight’s counterpart, Shadowlance. It is a 9* Humanoid monster that also has fantastic stats. Its moves are Eclipse, Doubleslash, Dusk, and Metal Slash.
Both recipes are obtained via quests very late in the storyline.