A-class debuff= Single buff/debuff, Just ATK-down or just DEF-down (Belobog,Oberon,Ambush)
B-class debuff= Both ATK-down and DEF-down (Cybele's,Sniper,Sneak-attack,etc)
C-class debuff= Special debuff and it stack with all, currently only Sol and Zephyrus have it
Eidolon buff/debuff= Stack with A,B,C.
Element buff/debuff= Stack with A,B,C, and Eidolon's buff/debuff (Light element atk-up like that)
A,B,C,Eidolon buff/debuff,Element buff/debuff stack. Bigger/better debuff skill with the same class or type will overwrites previous skill. But there's a limit of DEF-down status on an enemy (50% is max). The limit on buff... I dunno too eheh
edited a bit -_-'