Nikke - Memory of Goddess EX farm 

most of the weapons can be combined once it reach level 5, im not sure just how to combine boomerang, u can see if a weapon has a combination by looking right behind the 2nd weapon u choose, u see another weapon indicating that they cam be combined at level 5 both

I believe it says it when you go to the item catalogue (as it lists the location of all the items.)