As it would turn out, Toukiden is a well-balanced game for the most part. Every enemy has a weakness to one or two elements. For example, the Manhunter — the large spider boss which happens to be one of the first you encounter — is weak to fire, yet strong against water. That is good to know, because you can equip a fire weapon before you head out on a mission that will see you up against a Manhunter. But the right element is not always apparent, so it helps to have an oni weakness guide that explains it all.
The information can be gleaned by reading the records section before leaving on a mission. As always however, you may not be able to read that information if you have yet to encounter the boss. For that reason, I have included a simple oni weakness guide below that outlines what each boss is strong and/or weak against.
Simple Toukiden Oni weakness guide
Manhunter
Weak: Fire
Strong: Earth
Bloodhunter
Weak: Wind
Strong: Earth
Windshredder
Weak: Earth
Strong: Wind
Skyshredder
Weak: All except Sky
Strong: Sky
Jollux
Weak: Wind
Strong: Earth
Edax
Weak: Fire
Strong: Water
Pyropteryx
Weak: Water
Strong: Fire
Astroteryx
Weak: Earth
Terragrinder
Weak: Wind, Fire
Strong: Earth
Pyrogrinder
Weak: Water, Wind
Strong: Fire
Harrowhalf
Weak: Sky
Strong: Fire, Water
Dreadhalf
Weak: Sky
Strong: Earth
Viper Queen
Weak: Earth
Strong: Water
Viper Queen (Kagachime)
Weak: Earth
Blade Wing
Weak: Water
Strong: Earth
Blade Wing (Inkarura)
Weak: Earth
Chtonian Fiend
Weak: Water, Wind
Strong: Fire
Chtonian Fiend (Daitenma)
Weak: Earth
Hell’s Warden
Weak: Earth
Strong: Water
Hell’s Sentinel
Weak: Sky
Nightblade
Weak: Fire
Strong: Sky, Wind
Nightblade (Yomitosae)
Weak: Water
Cimmerian King
Weak: Water
Strong: Fire
Cimmerian King (Tokoyonookimi)
Weak: Earth, Water
That’s about it. You might want to bookmark this oni weakness guide if you’re playing through the game currently and will be checking back frequently. Best of luck hunters!