This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
Milon: Fire, Fir-2, Fir-3, Blaze (Edge), Jinn (Rydia), Nuke.
Kainazzo: Lit, Lit-2, Lit-3, Blitz (Edge), Indra (Rydia).
Valvalis: White, Lit-3.
Rubicant: Ice, Ice-2, Ice-3, Flood (Edge), Leviathan (Rydia), Shiva
* = Rubicant's weakness is only revealed when he is attacking. When
Rubicant is cloaked, all spells used against him will heal him instead
of hurting him.