help with chain devil encounter

hello! my party is made of 3 level 5 players. phoenix sorcerer mountain dwarf, triton banerett fighter, and fairy hunter ranger. they recently released a chain devil to the world and they have to go hunt it down and kill it.

basically i need help knowing what level they should fight it at, and maybe help building an encounter. the chain devil is recruiting other monsters to help him grow power and cause problems and whatnot, so really any monsters would work.

if anyone has any balancing resources, that would be great too.