First off, ask yourself 'what' your trying to fuse.

Shin Megami Tensei Imagines Demonic Compendium on the Wiki page will be used in my explanation.


From the Wiki Compendium you are free to choose your Demons type-

Beast
Demigod
Dragon King
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Lets pretend you are trying to fuse a Succubus.

In order to fuse a Succubus you will need a Yoma and a Tyrant whose combined levels are between 62-73. Dis (level 23) and King Frost (40).
40+ 23 = 63 (combined level) = Succubus



There are multiple ways to fuse almost all demons, experimentation is the key!

Happy Fusing Gaia/Megaten!